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THE PLANETS:
SOLVED AND UNSOLVED PROBLEMS
by D, Ya. Martynov
"Nauka" Press, Moscow, 1970
NATIONAL AERONAUTICS AND SPACE ADMINISTI^ATION - WASHINGTON, D. C. . MAY 1972
TECH LfBRARY KAFB, NM
NASA TT F-698
THE PLANETS: SOLVED AND UNSOLVED PROBLEMS
By D. Ya. Martynov
Translation of "Planety: Reshennyye i
Nereshennyye Problemy."
"Nauka" Press, Moscow, 1970
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
For sale by the National Technical Information Sen/ice, Springfield, Virginia 22151
$3.00
lllllllllll I I I I nil I I I Ml II nil I
TABLE OF CONTENTS
PAGE
1. INTRODUCTION 1
2. PLANETARY SURFACES 7
3. PLANETARY ATMOSPHERES 25
4. INTERNAL STRUCTURE OF THE PLANETS 41
5. INVESTIGATION PROCEDURES AND POINTS OF APPLICATION 53
TABLES OF PHYSICAL CHARACTERISTICS OF THE MAJOR PLANETS
AND THE MOON 72
MERCURY 72
VENUS 74
EARTH 76
MARS 78
JUPITER 80
SATURN 82
RINGS OF SATURN 84
URANUS 85
NEPTUNE 87
PLUTO 89
THE MOON 90
iii
THE PLANETS: SOLVED AND UNSOLVED PROBLEMS
D. Ya. Martjmov
ABSTRACT. A survey of the resolved and unresolved
problems of planetary physics is presented. The con-
tribution of worldwide research in the fields of
astronomy, surface astronomy, space technology and other
fields to planetary physics is reviewed. A list of
physical constants for all planets and their satellites
is given.
*
1- INTRODUCTION
Since the planets of the solar system have become the topic of space /_3
investigations, interest in them has grown to an extraordinary degree. Studies
have begun to be made both from spacecraft flying near them and by scientific
equipment in direct contact with the atmosphere or ionosphere of a planet and
even with its surface.
Planetary research has vmdergone a development which can only be de-
scribed as explosive. Only this explosion is creative rather than destruc-
tive. The size and pace of the investigations, which in the quite recent
past have been purely academic, have given way to extensive experiments and
to ardent, sometimes hasty discussions of the results extracted. The
approaching possibility of interplanetary travel has had a mysterious, but
very active effect on all these events.
*
Numbers in the margin indicate pagination in the original foreign text,
Astronomers, design engineers, scientists representing related sciences,
and simply inquisitive persons, are all showing interest. Correspondingly
the number of scientific investigations has grown and extensive new informa-
tion, based on the observations, has appeared. An avalanche of new facts
has fallen upon us, some important, and some minor, some substantial and
others secondary, some true and some doubtful.
Just as for a ship at sea or an airplane in the air, the most hazardous
part of the voyage for an interplanetary craft is the beginning and the end.
But in transit the craft is also confronted with a difficult problem, to
maintain a proper course and not deviate from it. Engineers designing space
rockets and interplanetary scientific stations and ballistics engineers who
determine the motion of the rockets at the beginning, enroute, and at the
finish, have all turned for information to astronomy. All previous accom-
plishments in astronomy have been mobilized to solve what is essentially an
engineering problem — that is, to move a spacecraft toward a given target.
Here precise knowledge is needed on the structure of the solar system and
its dynamics, and a linear scale of interplanetary distances is needed in
order to accurately utilize the precepts of celestial mechanics. These
methods are used to determine the rocket's trajectory. The observational
procedures of astronomy are also needed, as are the facts already extracted /4
concerning the physical nature of planets and interplanetary space.
All these achievements of classical astronomy were mobilized to serve
the new problems created by mankind. Astronomy, this most ancient of
sciences, is still the center of attention even today. Only now the re-
quirements imposed on accuracy of the answers are immeasurably greater than
before. The responsibility of astronomers has grown to include forecasting
the conditions involved in the motion of spacecraft, their launching and
landing. On the other hand, the landing of a space missile equipped with
scientific equipment, or even its orbit near a planet, as the successful
fligjits of the Soviet and American unmanned interplanetary stations showed,
will return a huge amount of highly reliable new facts. The success of these
flights has been tremendous. It has generated the Idea that the surface study
of planets is henceforth a thing of the past, and must give way completely to
space technology methods.
A tragic delusion! Considerable time must yet pass before even the
nearest planets can be studied exclusively with spacecraft, and the more
distant ones will be studied for many years to come by surface methods that
only will be developed and refined along with the space technology methods.
It is these experiments, that is, those on the surface of the Earth, that will,
to a significant degree, determine the topics of space experiments involved
in studying the planets.
The situation has not changed even following the latest success in
astronautics, that is, the landing on the Moon by the crew of "Apollo-11"
and its return to Earth carrying rock samples from the lunar surface, miles of
photographic film and results of observations carried out on the surface of
the Moon for a period of two hours, while the instruments which they left on
the Moon have continued to carry out the experiments set up there. The crew
of the "Apollo-11" brought to Earth new information which is distinguished
by its high degree of reliability, but how little this is in comparison with
the knowledge we must have about the entire Moon and all of its geographic
(or more precisely, its "selenographic") and physical properties! This will
all be possible only after trips to the Moon have become regular scheduled
flights.
Then does planetary research involve only the concept of interplanetary /5
travel? The answer to this question can only be a negative one. The primary,
and even ultimate, goal of any natural science is that of solving the origin
and development of the topics and phenomena to be studied. And astronomy,
as a subdivision of planetary research, has the fundamental problem of
explaining how and when our planetary system came into being around the Sun,
how it evolved, and how it will be in the future. To solve this problem, it
is essential to have knowledge about the planets: In addition to the usual
characteristics such as mass, size, form, and rotation period, it is necessary
to know the structure and chemical composition of the surface of a planet, ^
its temperature, as well as the temperature of the atmosphere, and the
qualitative and quantitative composition of the atmosphere. Attempts must
be made to discover the internal structure of the planets and the relationship
between planetary matter and comets, meteors and other interplanetary matter,
based on the external characteristics.
The study of the dynamics of planetary atmospheres greatly contributes
to the field of climatology, since it reveals the possibility of imderstanding
the motions characteristic of planetary atmospheres, as well as motions
unknown on Earth.
All this may be studied from the Earth's surface with great success,
and, in fact, is. being studied by astronomers. But space technology pro-
cedures are of Invaluable assistance. However, let us not cherish the vain
hope of expecting in the near future, "at last". . ."from now on...", "now — ",
etc. , that we shall know everything we wish to know about the planets, their
formation, and their evolution. Being on Earth, we are developing a whole
branch of science — geophysics — dedicated to the Earth itself, but we are
infinitely far from being satisfied with the knowledge gainedl As yet we
have no widely accepted concept as to the bowels of the Earth, its chemical
composition and temperature at great depths. We can not even solve beyond
doubt the question as to whether the origin of petroleum is cosmic or organic,
and we still often err in the forecasting of weather. The list of such
unresolved problems could continue without bound.
In the present booklet we wish to give a survey of the resolved and
imresolved problems in planetary physics that are of the greatest Interest
today. We must say that these do not always coincide. For example, the
degree to which the surface layer of a planet is friable is interesting, both
theoretically and practically. Theoretically, because to some degree It
determines the temperature conditions of the surface layers of the planet,
and practically, because the conditions for landing a spacecraft depend on
it. But in the first Instance the astronomer is interested in the nature of
- ■*-'Jti
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^ii^£^
N- -a
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Figure 1. First men on the Moon. Expedition
of the space ship "Apollo-11". Landing
module; "Eagle" on the Moon, July 21, 1969.
Astronaut Aldrin near seismometer he set
up, equipped with solar batteries. Left
center shows instrument for reflecting
laser beam. Rear part shows imprint of
Aldrin' s boots on the Itmar soil.
the surface layer, its petro-
graphic structure, thermal
conductivity, etc., and in the
second instance he is concerned
with its capacity to withstand
a dynamic and static load. The
answer to the practical question
of strength may be given with /7
the answer to the first — a
theoretically significant
question — but, unfortunately,
it is far from having that
degree of reliability and
accuracy which the designer
must have. It is not surprising
that the paths of astronomical
and design problems often co-
incide in the area of planetary
research, although the means
for solving them are not always
identical.
In this survey, we shall not give a systematic discussion of the facts
and methods employed in planetary research — this is the concern of text-
books. We shall speak about the latest successes and achievements in studying
the planets, making reference to the surface methods, since space technology
methods are quite widely known. And for this same reason the Moon will not
be a matter of special concern, because formally the Moon is not a planet,
but a satellite. The Moon is becoming an ever increasing topic of direct
investigation. But any systematic investigation of the Moon by such means
is a long way off I
With this booklet, the author addresses himself neither to the reader
v*io is a specialist in astronomy, nor to the reader who is making his first
steps in planetary astronomy, but rather to the reader who has general
knowledge as to the current state of the art In planetary science. The
present essay may perhaps permit the reader to evaluate the degree of
reliability of his knowledge and direct his attention to those fundamental
problems which are still to be solved by the methods of surface astronomy.
''• PLANETARY SURFACES
Our knowledge of planetary surfaces is limited to the inner planets, or /8
planets of the Earth group as we still call them — that is. Mercury, Venus,
and Mars. We can also include the Moon among them. On Jupiter, for example,
all that we can see are atmospheric and cloud formations. It would appear
that we might say the same with respect to Venus, since we can see only its
cloud layer, which is very thick and has almost no opening. But in quite
recent times, by using radar impulses, it has been possible to penetrate
the atmosphere of Venus and, reflected from the solid surface, to return to
Earth the first information on the various formations on the planet's surface.
It has now become possible to compile the first schematic map of Venus where
individual features are shown, although it is true these features have as yet
not been interpreted. But the map of Mercury, compiled from visual telescopic
observations (Figure 2) , contains only dark formations on a light background,
the nature of which is completely unknown.
Ultimately, it is the observations from a planet's surface or stable
atmospheric formations that will permit the period of rotation around its
axis to be determined. This period, in conjtmction with the mass, dimensions,
and shape of the planet, will offer the first signs of its internal structure.
Along with this, the alignment of the axis of rotation is derived from such
observations and it becomes possible to map the planet.
At the present time, we already have at our disposal the correct values
for the periods of rotation of all the major planets including Mercury and
Venus, the data for which were obtained only by using radar (see page 54
for details). The 59-day period, found for Mercury, indicates an angular
velocity of rotation equal to the angular velocity of its motion around the
Sun at perihelion, that is, on that orbital segment which is nearest the Sun.
Tidal forces have apparently played a role in establishing such an equation.
T
^° a?* &r f20' m" 130° 210" 2W 170' Ja7°. 3S0°
Figure 2. Map of the surface of Mercury, com-
piled by 0. Dolfuss from observations at the
observatory Pic du Midi (France) .
The 243-day period of
Venus' retrograde rotation is
coupled with the rotation
period of Venus and Earth
around the Sun, such that
at each minor conjimction of
Venus — that is, when Venus
is located between Earth and
Sun — the same side of Venus £9^
is turned toward Earth. The
Earth occupies the same
position above the horizon
from some point on the surface of Venus every 146 days, and the minor con-
junctions of Venus are repeated every 584 = 4 x 146 days. As to whether this
alignment is random or not, we still do not know.
It is interesting that the previous visual observations of Mercury
resulted in a period of rotation around its axis of 88 days, equal to the
period of its rotation around the Sun. It was found that Mercury has the
same side always turned toward the Sun. This conclusion is no longer valid,
but all the ancient drawings of Mercury's surface which previously served for
its mapping with an 88-day period, surprisingly enough, are satisfactorily
encompassed by the new period of 59 days. It seems that such a situation is
due to in determinancy in the drawings, which in turn, is due to the problems
involved in observing Mercury.
We have today maps of the surface of Mercury and of Mars. That of
Mercury is very crude, but the map of Mars contains numerous details, the
comparison of which at different oppositions indicates a substantial time-
variability in the face of Mars, not only due to variations in time of year
(Martian), but independently of them, as well.
Of course, the most detailed maps are these of the Moon; global maps /lO
have been com.piled in scales of 1:5,000,000 and 1:10,000,000. For individual
8
Figure 3. Map of the surface of Mars In
oppositions of 1956 and 1958. South -
above. In 1956 Mars had mainly its south-
ern hemisphere turned to the Earth. On
the 1958 map, the aerographic latitudes
are shown on the left and the longitudes
at the bottom.
regions, larger scale maps
have been compiled which were
produced by the flights of
spacecraft around the Moon.
A map of the equatorial zone
has been compiled for the
visible side of the Moon in a
scale of 1:1,000,000. Here
terrestrial and space inves-
tigations successfully work
together: to compile large-
scale maps with the aid of
highly informative photographs
taken at close distances, a
large number of reference
points is required, the
position of which is determined
on the surface of the Moon
(selenographic coordinates) from ground observations, connecting the reference
points with the circumference of the lunar disk and thus with its center.
To map the dark side of the Moon only space methods can be used, and
cartographic continuity requires photography which will Include part of the
visible side of the lunar surface and part of its dark side in a single frame, /ll
Naturally, it is those points on the visible side which are found near the
edge of the lunar disk in observations from Earth that are photographed in
such an instance. Determination of the selenographic longitude for these
points may involve considerable errors, which then cause the longitudes of
features on the dark side to be in error. Because of this, it was found
that the longitudes determined from data of the Soviet and American space-
craft (Luna-3, Zond-3 and Lunar Orbiter) differed by up to 7°. In linear
dimensions, this represents 200 kilometers. Of course, such discrepancies
are not allowable, and they were eliminated in further investigations.
l«i<C\f !3---.-i--;L*-. -=-,•,.••.■ '■■--■ -.'-^^ t.;' •>'..,-. - ■ , '.-
1
Figure 4. Part of the surface of Mars at a
distance of 3500 km. Photograph taken by
the unmanned spacecraft Mariner-6.
The achievements of the
unmanned interplanetary sta-
tions of the "Mariner" series
(Nos. 4, 6, and 7), which
revealed numerous circular
mountains very similar to such
formations on the Moon, have /12
forcibly advanced problems which
we call geomorphological prob-
lems with respect to Earth.
The morphology of a planetary
surface reflects its past
history. The similarity in
the surfaces of Mars and the
Moon, of course, indicates a
similarity of formation.
Their differences, caused
mainly by the existence of an atmosphere on Mars and the lack of one on the
Moon, are of considerable interest. The total number of circular formations
per square kilometer of the surface of Mars is the same as on the lunar
continents, although small formations with diameters from 20 to 3 km are more
numerous on the Moon. Since the formations that are less than 3 km in size
are not distinguishable on the Mariner-4 photographs and not all objects with
dimensions greater than 3 km (approximately up to 10 km) can be recognized
on these photographs, we can only guess as to whether their small number
is a result of observational selection, or is a result of their obliteration
under the influence of winds, distortion from meteoritic impacts, or by
thermal stresses. With Mars being near a ring of asteroids, we might expect
a considerably greater number of circular mountains on it, but if this is
not true and the efficiency of the above landscape obliteration is unknown,
then the hypothesis of an endogenous or volcanic origin of the craters
becomes plausible • No matter what the true situation is, all the detailed
photographs of the Moon at close distance only strengthened the opinions of
those who considered that the lunar landscape was formed under the action
10
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^1^^
diidliSiJS
i;:^-.-*^, j4a.iy.'tff 'a;i^::,a^^>■•a:
iff-"'^^
Figure 5. Mars at a distance of 90
million km (Pic du Midi, France,
1967) and 920,000 km, July 29, 1969
(Mariner- 6, below) .
of both internal effects (vulcanism,
tectonics) and external effects
(incidence of meteoric bodies) .
The different radioactivity of
the various lunar formations obtained
in the lunar orbits of our unmanned
space stations Luna-10 and Luna-12
speaks in favor of the first hypo-
thesis. This difference indicates
that basic rocks predominate in the
seas (basalts) and ultrabasic rocks
on the continents. The first probably
has a high amount of iron (the so-
called ferrobasalts) . As a whole,
as chemical analysis of the lunar
soil made by the Surveyor, the Luna-9,
and the Luna-13 showed, the lunar
surface rocks are the result of fusion.
Analysis of the composition of the
samples collected at the landing site
of the Apollo-11 in the Sea of
Tranquility revealed a rather high
petrographic diversity and, in general,
a significant similarity to igneous /13
rocks on Earth, if we do not consider /14
the very high titanium (TiO„ up 10%
by weight), Zr, Y, and Cr content
with a significant sparsity of alkali
metals Na, K, and Rb. These volcanic specimens contain numerous gas cavities,
about 50% clinopyroxene, about 30% plagioclase, a large amount (up to 15%) of
ilmenite and granular impregnations of olivine, and sometimes iron-nickel
spheres. These can be included in the olivine-containing basalts. But there
11
are basalt specimens which contain no olivine. Under terrestrial conditions,
basalt lavas are smoothly extruded.
Along with the basalts, the Investigated lunar specimens contain
breccia type rocks which consist of cohesive angular fragments less than 0.5
cm In size. Traces of microfractures and numerous vitreous Inclusions can
be seen here. Their structure Is commensurate with the thesis that they
were formed by the powerful Impact of a body on the lunar surface, falling
onto the Moon from outer space.
In general, the rocks collected on the Moon reveal traces of erosion
(Impact?) on the upper surface, whereas the lower surface has apparently
remained unchanged. Analysis of the age of specimens from the Sea of Tran-
quility (for the potassium: argon ratio) indicates that they were crystallized:
from three to four billion years ago; that is, they are older than the most
ancient of terrestrial rocks. According to the traces which cosmic rays have
left in them, these specimens had been at a depth of more than a meter under
the surface during their entire existence except for the last 20-160 million
years; that is, they were ejected to the surface either as a result of
meteorltic erosion or as a result of tectonic processes.
The fact that the upper cover of the Moon still lives and "breathes"
today is Indicated also by the optical phenomena repeatedly mentioned by
observers during the time of telescopic observations of the Moon (such
phenomena are numbered at about 600 Jover three-and-one-half centuries) and
especially the generation of gases in the region of the crater Alphonsus
mentioned by N. A. Kozyrev in 1958 during spectral observations. In order
to reliably answer the question of the authenticity of rapid changes on the
surface of a planet or of the Moon, we must have simultaneous observations at
several astronomical stations separated from one another in longitude by
90-120° and directly connected with one another by modern communication /15
facilities.
12
We should mention that the
combined influence of the internal
and external factors in the
formation of the lunar landscape
(just as for Mars) is apparently
inescapable, since the impact of
a large meteorite will arouse
volcanic activity in that region.
The number of craters on the
Moon is so high that they can
never be considered to be only
Figure 6. Surface of the Moon, taken at a the result of the impact of
close distance with a television camera
on the unmanned spacecraft Luna-9.
asteroids and large meteorites,
which occurs rather rarely. It
would be more proper to credit the formation of the craters to that era when
the Moon was just forming and many plane tesimals, as yet unabsorbed by the
planets, were moving around it. When the Moon absorbed them, circular
mountains of various dimensions were formed. Gradually a state of saturation
was established, when any new impact would annihilate the older craters and
create new ones. In fact from one (in time) impact several ejections may
occur which either form a chain of several craters, or only one which is
elongated in the direction of the ejections — as, for example, the crater
Schiller in the southwest part of the visible lunar disk. The large number of
small craters arranged around the crater Copernicus are undoubtedly of
secondary origin. They were formed as a result of the explosive dispersion
of lunar matter when the basic crater was formed.
J16
These craters can be easily discerned on the dark background of the
Oceanus Procellarum. This same dark coloration of a different degree of
saturation is possessed by all the seas on the Moon, their surface is
comparatively smooth, and they are either devoid of craters, or have very
few. The reason for this sparslty is the age of the seas. These are young
formations, formed about two billion years ago, on which numerous large
13
bodies, preserved until the later stages of development of the solar system,
left traces of the impact. However, in the very youngest formations, such
as, for example, the craters Tycho or Aristarchus, the surface is very
uneven and covered with large fragments. With time, they will be broken up
and acquire a fine structure on an overall smooth background, and only
massive impacts will disturb this picture. New impacts will disturb the
already existing formations by different means, that is, by direct impact
or by secondary Impacts during dispersion. The lunar surface has undergone
events of such nature many times during its existence, and very ancient
objects have gradually disappeared. Only the largest of them, many hundreds
of kilometers in thickness, can still be traced in our time.
Although on the Moon, in contrast to the Earth, the changes on the
surface take place with extreme slowness because of the absence of weathering
and erosion, the Moon, nevertheless, is not a museum. It is externally
vulnerable, and the effect of external factors continuously changes its face.
Therefore, we can understand that those lunar formations, whose age exceeds
four and one half billion years, have not been preserved up to the
present.
Craters are also encountered on the Moon which are similar to volcanoes
on the Earth (around this same Copernicus) , and sometimes they are quite
numerous. It is assumed that at times lava flows from them, but these
processes are of a local nature and determine the lunar landscape to only a
small degree.
The soft landing on the Moon of the unmanned space stations, Luna-9, /17
Luna-13, and the Surveyors, not only gave information on the chemical com-
position of the lunar cover, but also established its macroscopic structure,
that is, finely crushed bunches, dust particles (about 10 y) , in which small
and large fragments and basic lunar rocks are imbedded. Contrary to former
concepts, they are weakly bonded and by no means form strong strata, con-
sisting of minute grains. Very fine dust, in fact, does cover everything on
the Moon — the finely crumbled surface and rocks, but the supports of the
14
"Eagle" (see Figure 1) sank only 5-7 cm Into the ground and the astronauts'
feet sank only several millimeters. In an attempt to sample depth, a steel
tube was sunk 5-7 cm without difficulty, and only with a great exertion of
force was a depth of 20 cm reached, but the core samples taken were found to
be finely structured. The crew of Apollo-12, which landed in Oceanus
Procellarum, encountered a more friable surface layer. The density of the
friable surface material of the Moon, as the samples showed, was altogether
3
0.8 g/cm , and beneath them, starting at a depth of 5-10 cm, the density
3
reached 1.5 g/cm . Finally, the density of the rocks and stones lying on
3
the surface was equal to approximately 2.8 g/cm . As radio observations of
the Moon show, the density of its surface layer gradually increases with
depth and reaches the density of the underlying rocks at a depth of one to
several meters. The surface layer is thicker in the seas — up to ten
meters. With all the similarity between the Martian and lunar landscapes,
we can hardly expect the same surface structure on Mars. The existence of an
atmosphere makes it impossible for the dust particles to adhere, as this is
inevitable in a vacuum. The surface of Mars must have a more friable
structure, and this is confirmed by radar observations which indicate an
incomparably greater smoothness on it than on the Moon (see page 17).
If it has become possible to obtain information, using unmanned space
stations, on the chemical composition and also to study the structure of the
upper cover of the Moon, then its petrographic compositions are derived
mainly on the basis of analogies from photometric, spectrophotometric, and
polarization observations. These analogies in the majority of cases are /18
loose, since the result is usually ambiguous, and, furthermore, the color /19
varies depending on the degree to which the matter is broken up and depending
on the ultraviolet radiation or proton radiation. So, for example, the
almost universally accepted conviction that the surface of Mars is composed
of limonite (pe -nH^O), based on the similarity of many spectrophotometric
and polarization characteristics, is encountering difficulty today, since
with advanced investigations new characteristics are being discovered that
do not coincide for Mars and for limonites.
15
No less ambiguous are the
results of radio obseirvations
of the natural thermal emission
from the planet in various
ranges . Emission at long waves
comes from the subsurface strata.
The phase lag in the maximal or
minimal temperature and the
variation in the mean diurnal
(on the Moon — monthly) tempera-
ture with wavelength (or what
amounts to the same thing, with
depth) gives the possibility of
judging the thermal conditions
of matter on the planet's
surface layer at various depths,
and its thermal and electrical
properties. Unfortunately, only
the rather complicated quantity,
the coefficient of thermal
-1/2
conductivity y = (XP^) > is
amenable to direct determination;
this coefficient connects the
coefficient of thermal conduc-
tivity x> density p and specific
heat c of the material. As yet
this method has given specific
information only about the Moon. Averaging which is too broad is obtained for
the planets over practically the entire hemisphere of the planet; it is
difficult to eliminate the influence of the atmosphere. In its application
to the Moon, this method gave a result that is in accordance with other
methods; its surface is composed of finely crushed rocks. The temperature
gradient is sizeable, thus indicating a heat flux from the depths of the
Moon. Using reasonable values for two of the three quantities X» P> c, we
Figure 7. Large (0.5 m) rock on the surface
of the Moon (Photograph taken by the un-
manned spacecraft Surveyor-1) .
16
can find the third and then seek rocks which have the appropriate properties.
However, in this case there is still a wealth of choices to be made.
Narrowing the range of solutions is only possible after a systematic
direct investigation of the various sites on the lunar surface becomes
possible with the aid of spacecraft that have landed there, including manned
craft. Comparison of the radio observations and the results of direct
analyses will make it possible to standardize the radio observations, that
is, to establish a correlation between real rocks and their radio emission. /20
Later this standardization can be applied to an analysis of the radio ob-
servations from Mars and Mercury. The use of radar methods has been of
substantial aid for this purpose; these methods make it possible to study the
reflectivity of the component rocks.
The radar technique is effective also in solving the question of the
degree of roughness of the planetary surface. Two mechanisms operate in the
reflection of radio waves, that is, quasimirror reflection from large-scale
irregularities (without disruption of coherence) and disordered scattering
on small-scale heterogeneities, whose dimensions are on the order of magnitude
of the wavelength. The first is performed along the normal, since the
transmitting and receiving stations either coincide or may be spaced over
the Earth, which even from the Moon is visible at an angle less than 2° . If
the slopes on the planet are generally not high, coherent reflection will
take place only from a small region in the center of the disk, such that the
site of the reflection may be established with complete confidence. Scattering
on small surfaces is disordered, strongly damped and unpolarized, which
permits distinguishing it without difficulty. On a comparatively rough
scale (A ~ 70 cm) the smooth slopes are 3° for Mars, 6° for Venus, and 10°
for the Moon and Mercury. As applied to Mars, the photometrically processed
photographs obtained by Mariner-4 led to the conclusion, from fluctuations
We mean here the inclinations of the lateral slopes of the surface
formations toward the horizontal plane.
17
Figure 8. Radar picture of the
vicinity of the lunar crater Tycho,
taken at the radioastronomy ob-
servatory at Haystack. The picture
permits distinguishing details
having a dimension of 1 km. Unlike
an optical picture, the details
here are differentiated from one
another based on their ability to
reflect the radio signal sent from
the Earth (wavelength of 3.8 cm).
Not only is the reflectivity in
Itself manifested, but also the
slopes of the reflecting elements
to the line of sight, since large
details have a specular reflection
and if the corresponding surface
is not perpendicular to the inci-
dent beam, it is either not
reflected back in the same direc-
tion, or a weak diffusely reflected
signal will be obtained.
in brightness as a function of the
angle of elevation of the Sun, that
on a scale of 3 km and higher the
slopes in the irregularities lie in
the range of 1-3°, although on the
slopes of the craters. Inclinations
of up to 12° are possible.
Polarization of radio waves,
reflected from the edge of the planet's
disk, as well as from its natural
radio emission, makes it possible to
determine the dielectric constant of
the planetary matter and thus narrow
the scanning range for the material
comprising the surface.
As applied to the Moon, the
complllation of the radar chart was
found to be fully successful. At a
wavelength of X = 3.8 cm, a resolving
power on the order of 1 km was
attained, which approaches the re-
solving power of optical astronomy.
Figure 8 shows such a map, compiled
at the Haystack station (USA) and
showing the distribution of the
reflectivity (by no means light and /21
dark) in the region of the crater
Tycho. In order to obtain similar
results with respect to the planets, much more powerful telescopes are re-
quired (at least two orders of magnitude larger) , since the planets are two
orders of magnitude farther away than the Moon; correspondingly the direc-
tionality of the radio antenna must be raised by two orders. In this case
18
the same strength of the reflected signal will not he attained; this signal
grows only in proportion to the root of the fourth power of the antenna /22
cross section. Even today, the sensitivity of the apparatus used in radar
is admirable; the radio impulse reflected by Venus is so weak that it is
impossible to find it directly. It is discernable only by complicated
computational analysis. Its energy is similar to the work of one step of a
mosquito.
Mars, whose surface has been most studied after the Moon, leaves a
number of unresolved problems, among which the important ones are the
questions concerning the nature of the polar caps and the cause of seasonal
variations in them. In this connection, we have not mentioned the nature of
the canals on Mars, since after almost one hundred years of discussion, the
"public opinion" of the astronomers tends to believe that the canals are the
result of schematizatlon in drawings (or examining photographs) of Mars.
It is an involuntary geometrization introduced by the observer into the
picture of the distribution of extremely weak and unclear details.
After obtaining photographs of Mars at close distances with the aid of
the unmanned spacecraft Mariner-4, Mariner-6 and Mariner-7, which revealed
no signs of such canals, the question as to the canals on Mars can be
considered "closed".
As we know, the atmosphere of Mars contains water vapor in a small
amount, and mainly carbon dioxide (see below). This gives a basis for assuming
that the polar caps of Mars consist of snow or of solid carbon dioxide.
The temperature of the Martian surface at the poles allows either assump-
tion. The spectral observations favor snow. In the light reflected from it
there are absorption bands with a wavelength of about 1.4 and 1.9 y. The
same is observed in the spectrum of the polar caps of Mars. The character of
the light polarization reflected from the polar caps of Mars is the same as
in hoarfrost which is formed at low temperatures by means of direct conver-
sion of the water vapors into the solid state. The reverse process, taking
place with heating of hoarfrost, also leads to sublimation from the
19
solid state, without thawing into the gaseous state, and the remaining
hoarfrost assumes a porous structure. Its very weak polarization is similar
to that observed at the polar caps of Mars.
In addition to this, we must remember that in the Martian atmosphere
carbon dioxide is predominant and the very low temperature at the Martian
poles (about 150° K) does not permit the carbon dioxide to remain in a gaseous /23
state. Precipitation of the overwhelming part of it in the form of "snow"
and dry ice Is unavoidable. Therefore, the main component of the polar caps
of Mars is carbon dioxide mixed with water in the solid phase. A very small
admixture of snow is necessary in order that significant bands are found in the
reflected light with wavelengths of about 1.4 and 1.9 y.
As far as seasonal variations in the dark "seas" of Mars are concerned,
which have basically the same color as the light "continents", only with a
lesser albedo, as we well know they are often attributed to the growth of
plants with onset of the warm season, melting of snow, and moistening of the
soil. Included in the sources of moisture, we have mentioned mineral water
since all of Mars in terms of its mean temperature (see below) is located in
a state of permafrost, thawing only at the top and for a short time in the
middle and lower latitudes. It is difficult to call these processes anything
other than biological processes which might vary the structure of the upper
cover of the planet's surface as a function of time of year. In the absence
of, or extreme sparslty of, free oxygen on Mars, vegetation there may exist
only in the simplest forms. Spectral investigations, unfortunately, have not
helped solve this question, but the polarization investigations which
indicate, first of all, the structure of the reflecting surface foster the
assumption of mass breeding of small organisms in the form of opaque granules
Including sporous plants — algae, Cetrarla caccullata, and fungi. The
precipitation of crystal formations would give a completely different
polarization picture.
The extraordinarily low contrast of the individual segments on the
photographs of Mars, obtained by Mariner-4, fully correspond to the picture
20
long-mentioned by Earth observers, that Is, the extreme "grayness" and the
insignificance of the details on Mars during the winter season. Marlner-4,
orbiting Mars, photographed primarily its southern hemisphere, when winter
prevailed there. Mariner-6 and Marlner-7 were in the most favorable position
— their photographs were much better: Marlner-6 photographed mainly the
northern hemisphere where at this time It was early fall, and Marlner-7
photographed the southern hemisphere where it was early spring.
Radar observations of long-range reflection of the emitted signal ,„,
reliably revealed the irregularity of the relief, reaching 12 km on Mars.
We should not be surprised at this. The differential in heights on the small
Moon is not any less. If the depressions on Earth were not filled with the
ocean, the height differential externally observable between the Himalayas
and the Phillipine trough would reach almost 20 km. But on Venus, radar did
not reveal any altitude difference greater than 2 km. It is true this refers
to the topography along a certain parallel, for which the Earth was at
zenith during the observations. On other parallels, the situation may be
completely different I
In finishing this examination of the questions involving the structure
of planetary surfaces, we should emphasize that the tempting possibility of
making an analogy here with the structure and composition of meteorites is
dangerous, since it is quite doubtful that meteorites are fragments of large
planets. They have never been subjected either to the effect of very high
pressures or to that of high temperatures.
Determination of the absolute values of the temperature of the planet's
surface Is very specific. Optical methods based on measurements in the
Infrared band of the thermal flux, coming to us from the planet, give a
comparatively high degree of resolution by cross section. In particular, on
the Moon numerous "hot" points have been detected which are revealed by slow
cooling at sunset or in the process of lunar eclipse — this is direct proof
of the absence of a thick heat-protective dust cover at these sites.
21
But only as applied to the Moon and Mercury which have no atmosphere
does this path lead directly to the target. As applied to Venus, the
measurements of thermal flux in infrared rays gave a temperature of about
240° K, referring to the upper troposphere rather than to the surface, and as
applied to Mars — a surface temperature, slightly distorted by atmospheric
influences. Measurements of thermal flux in the radio band are free of this
disadvantage to a much larger degree, but they are also not without fault.
This is because, in the centimeter and millimeter bands, the radio waves are
absorbed in the atmospheres of the planets, and in the decimeter and decameter
band the ionosphere, if it is sufficiently dense, may introduce disturbances.
It is just such a situation that was created in interpreting the /25
measurements of the radio emission originating from Venus. At wavelengths
from 3 to 10 cm, the measurements give a brightness temperature up to 700° K
and above, but at the shorter wavelengths it is much lower, below 300° K. At
the 21 cm wavelength, a drop in temperature is also observed. A ten-year
discussion accompanied by ever newer and newer measurements have led the
majority of astronomers to the conclusion that only the measurements in the
3-10 cm band pertain to the solid surface of the planet. The shorter waves
are absorbed strongly in the atmosphere; therefore, the temperature of 300° K
pertains to the upper atmospheric layers. However, only the direct tem-
perature measurements in the atmosphere of Venus carried out in the brilliant
experiment of the Soviet unmanned spacecraft Venera-4 — and the proofs thus
found that Venus has no significant ionosphere (it exists only on the day-
light side and is apparently caused by solar x-ray radiation) — finally
convinced everyone that the surface of Venus is in fact very hot, and placed
before theorists the problem of explaining this fact.
Measurements on the large radiotelescope at Pulkovo, the flight of the
American unmanned craft Mariner-2 near Venus and, finally, the interfero-
metric measurements with a high resolving power at the 10 cm wavelength have
all made it possible to study the temperature distribution across the disk
of Venus, although it is true it is only approximate and the results obtained
by various means are not without contradictions.
22
The temperature on Mars was found to equal approximately 200° K which
is the mean temperature across the disk, in a wide range of radio waves
(3 mm - 20 cm). It would be interesting to trace, by radio measurements
with high resolution, the existence of relationships between polarization of
the emission and the progressive seasonal darkening of the Martian seas,
visible during polarization observations in the visible band.
Measurements of the natural radiation from Mars in the infrared band
gave results similar to the results in the radio band, but not coinciding
with them: for those places on the equatorial belt of the planet's surface
where the Sun is at the zenith (subpolar region) a mid-day temperature was
found in the range from 250 to 280° K at aphelion, and about 300° K at
perihelion. Such differences should not be surprising, since the orbit of
Mars is rather eccentric and at its perihelion Mars is located significantly /26
nearer the Sun than at aphelion. The latest, more reliable measurements give
lower values, which correspond better to the radio measurements — for
example, for the entire illuminated hemisphere of Mars a mean temperature of
225° K (at aphelion) was found. Furthermore, the temperature of the Martian
surface undergoes very strong variations in the course of short days (ZA-tt
hours) . During the day there, shortly after noon, it reaches a maximum of
about +20° C; in the morning after the nocturnal radiation of heat into outer
space, the temperature drops to -60° C. The reason for such a rapid cooling
undoubtedly is due to the low density of the atmosphere (see below) that is
incapable of retaining the natural radiation of the planet. The latest
measurements by Mariner-6 revealed a new temperature of 150° K at the polar
caps on Mars .
New measurements of Mercury in the infrared band give a temperature of
620° K for the daylight side. The dark side is no hotter than 150° K, but
also is somewhat colder, which does not agree with the idea that there is no
atmosphere on the planet and with the new correct value for the period of
rotation, for which the solar days last 176 Earth days. In the event there
is no atmosphere, the temperature during the night would have to sink much
lower and then, in measuring the integral temperature across the disk of
23
Mercury, a notable phase effect would be observed — that is, a dependence of
the measured temperature on the value of the unllluminated (night) part of
the planet's disk. But the phase effect on the temperature of Mercury is
very weak.
24
■ ■■IIIIH 11 I II UriMIII
3. PLANETARY ATMOSPHERES,
The existence or the lack of an atmosphere is strongly manifested in the /27
many physical properties of the surface layers of a planet, that is, in its
thermal conditions, formation of the planet's landscape, possibility for the
evolution of life, etc. Furthermore, the chemical composition of a planet's
atmosphere gives signs as to the past history of the planet.
A planet's atmosphere is revealed by a fading of brightness toward the
edge of the disk, by blurring, by clouds, etc., when simple telescopic
examination of the planet is involved, but for quantitative determinations
we must have measurements — photometric, polarization, and spectral. The
photometric measurements are the simplest, but their interpretation will
not provide a completely unequivocal answer, because the presence of suspended
particles and aerosols in the atmosphere will somewhat complicate the theo-
retically clear photometric effects of a purely gaseous atmosphere. In
particular, it will lead to an exaggeration of the thickness of the atmospheric
layer. The same can be said of polarization measurements. Neither measurement
procedure will give any indication of the chemical composition of the atmos-
phere .
Incomparably more accurate and complete information is given by spectral
analysis, both in ground observations, and especially when receiving equipment
is launched into the stratosphere or completely beyond the confines of the
Earth's atmosphere. Still more information is given by chemical analysis of
the atmosphere carried out by equipment penetrating it, such as was done by
the unmanned craft Venera-4, Venera-5, and Venera-6.
A summary of our current knowledge on the chemical composition of
planetary atmospheres is given on Table 1 on page 26.
25
TABLE 1. GASES WHOSE PRESENCE HAS BEEN DETECTED IN
THE ATMOSPHERES OF PLANETS AND SATELLITES
Gases
Planet
Gases
CO2??
Planets
Uranus
Mercury
Venus
co^::, CO, n^, h^o.
Neptune
0^, HCI, HF
N^::, o^:, h^o, Ar,
Pluto
Earth
Satellites of
Jupiter
CO2 Ne, He, CH^, Kr,
N2O, H^, 0, 0^, Xe
CO^, CO, H^O, CO2
ion, H, C, atoms
Mars
Titan (satel-
lite of Saturn)
Triton (satel-
lite of Neptune)
Jupiter
H^, CH^, NH^
Saturn
H^, CH^, NH^C?)
^2' ^"4
H^, CH^
No data
No data
CH,
4
No data
There is every basis for assuming that a large amount of helium exists
in the atmosphere of Jupiter. But its resonance lines are found in the far
ultraviolet band, and the necessary excitation sources are lacking for the
appearance of nonresonance lines, taking place from the excited levels.
In precisely the same way, the existence of molecular nitrogen on Venus
would be impossible to establish by means of ground observations, since its
clearest spectral characteristics are found in the ultraviolet spectral band,
which are completely blocked by the Earth's atmosphere. The existence of N„
on Venus was first established by Venera-4, just as 0„ and H„0, which had
previously been detected only hypothetically.
/28
The unmanned space stations Venera-4, Venera-5, and Venera-6 reported
reliable quantitative data: the equipment installed on them showed that the
atmosphere of Venus is very hot, and in its composition carbon dioxide
occupies about 97 +4%. The gases N„, 0„, and HO must be assumed as merely
26
Impurities. The numerical data should be assumed as approximate, since it
is still not clear as to which level above the surface of the planet they
refer.
If we assume that Venera-4 discontinued transmission of information at an
altitude of about 20-25 km, it is precisely to this level of Venus' atmosphere
rather, than to its base, that we must attribute the measured values of T = 540°
K and P = 18.4 atm. Then by extrapolation, we can find, for the surface of
Venus, an atmospheric pressure near 100 atm and a temperature above 700° K
with no unusual fluctuations from day to night. Almost the same data were
reported by Venera-6: in the altitude range from 48 to 10-12 km the pressure
varied from 0.5 to 27 atm, and the temperature from 300 to 600° K. If these
data are extrapolated according to adiabatic law, then for the surface of
Venus we will obtain a pressure of 100 atm and a temperature of about 770° K.
Similar data are obtained on the basis of ground radioastronomical observa- /29
tions. The data from Venera-5 differed rather strongly from these results.
The reason for these deviations is still not clear. If we take the mean
value from those obtained by Venera-5 and Venera-6 for the amount of water
vapors equal to 0.05% over the atmosphere as an average, then their relative
content in the atmosphere of Venus is found to be the same as on Earth, but
the absolute value is found to be two orders of magnitude higher. However,
in the Earth's oceans the amount of water contained is five orders of magni-
tude greater than in the atmosphere. Therefore, we can confirm that the
amount of water on Venus is significantly less (by three orders of magnitude)
than on Earth. The same can be said also about oxygen. As far as the
absolute content of carbon dioxide in the Venusian atmosphere is concerned,
then it would approach in order of magnitude that which would exist in the
atmosphere of Earth if all its carbonate rocks were to release the C0„ bound
in them. To make a full comparison of the atmospheres of Earth and Venus,
we must know the amount of virgin C0„ and H„0, which is constantly leaving
the depths of the Earth into the atmosphere by volcanic eruptions. On the /30
whole, if the entire atmosphere of the Earth is assumed to be the result of
the generation of gases from the solid crust ("degassing") and we consider
27
V B IZ
2i m s
nnmTTrnrn-'niTiin Trr:rTi
ff ;■ \ .^ \ ff J _„_...
J L iiiL. :rii
.ji^i?
8710
_m-o
Figure 9. Spectrograms of Venus (upper) , Mars
(middle) , and the Sun (bottom) (taken through
the Earth's atmosphere) in the 8690-8730 A
band, occupied by a CO- carbon dioxide mole-
cule band, the lines of which (they are
scaled at the top) are very sharply visible
in the spectrum of Venus, much weaker in the
spectrum of Mars and totally invisible in the
spectrum of the Sun. The other strong lines
of the spectrum are formed either in the
atmosphere of the Sun or in the atmosphere of
the Earth.
that no chemical interaction
with the crust and the bio-
sphere has taken place, then
we find a composition that
is similar to that on Venus
(and Mars) with the excep-
tion of the excess of water
on Earth which requires a
separate explanation.
Thus, the atmosphere
of Venus also has a secondary
origin, but probably with no
biosphere. For the beginning
of life on Earth, a reducing
atmosphere was required (for
example, with an abundance
of methane CH,, hydrogen H.,
etc.). It would be inter-
esting to seek prebiologlcal
organic molecules in the reducing atmospheres of the major planets. It may be
o
that the absorptions at XA 2600 and 2100 A, detected in the spectrum of
Jupiter, have precisely such an origin.
Bearing in mind the difficulties in observing Venus from Earth, following
from the fact that it is a minor planet which is near the Earth only when its
dark side is mainly turned toward us, we must not exaggerate its cloud layer.
In fact, photographs of the Earth, obtained in the past two to three years
from the distance of the Moon to the Earth under favorable illumination
conditions, and also from satellites at closer distances, can be interpreted
by us only with difficulty and we are familiar with the map of the Earth,
since it seems to be so covered with a cloud cover. Although the albedo of
Venus is much greater than that of Earth, nevertheless, the veil of the
Venusian clouds is not dense, which is proved by the diversity in temperatures
28
found by spectroscopic measurement of the C0_ bands on Venus: the tempera-
tures are obtained in a wide range from 215 to 445° K, obviously depending on
how free of clouds is the site of the planetary disk that is spectrographi-
cally measured, and in the same manner at what great atmospheric depth the
observable spectral bands originate.
With the very slow rotation of Venus, its thick atmosphere must obey
forms of circulation that are completely dissimilar to those in the Earth's
atmosphere. Theoretical study of this phenomenon and its proof by observa-
tions are of the greatest interest. However, the observations of Venus have
established one type of atmospheric motion, similar to those on Earth, and
judging from the motion of the dark formations in the cloud layer of Venus, /31
the rotation period of the atmosphere at this level comprises four Earth
days in the same (inverse) direction as the rotation of the planet itself.
This indicates winds of great force in the upper troposphere of Venus, moving
at a velocity up to 100 m/sec In the direction of the planet's rotation.
Now we understand in rough outlines the atmosphere of Venus, but the
details remain very unclear. Including the nature of the cloud cover on /32
Venus and its role in the transport of radiation from the Sun to the surface
of the planet and back into space. Now that we have proved the existence of
water on Venus, due to the experiments with the unmanned spacecraft of the
Venera series, more than ever we can assume that the clouds on Venus are of
water. This is also Indicated by the temperature of the cloud layer ('\^30°C) .
But we have neither photometric nor spectroscopic proof of the aqueous nature
of the cloud layer on Venus .
Motions in the lower atmosphere of Venus where no instruments have yet
reached are completely unknown, as are the thermal conditions and Illumination.
Ground observations are in no position to answer this question, so that we
must place our hopes in future experiments with equipment launched into the
atmosphere of Venus.
29
,»3sr7?^?fr
tm-T"^:
Theory, by the way, does
permit answering one question,
which had formerly caused much
argument. This is the ability
of the Venusian atmosphere by
means of one of its effects,
(2)
the greenhouse effect , to
maintain a temperature of the
planet's surface and lower
atmosphere at a level of 700°
K or higher. Now the justifi-
cations for the greenhouse
effect on Venus have been
found. As soon as the huge
amount of carbon dioxide and
a sufficiently large amount of
water vapors on Venus were
proven, its high absorption
power in the near infrared
band of the spectrum was in a
position to retain the greater part of the natural radiation from the planet
and in the same manner maintain the temperature at a high level (see below.
Section 5) . There is no need for either a large influx of internal heat from
the planet or even of strong volcanic processes (they are accompanied by the
generation of large amounts of sulfur dioxide SO , but this gas has not been
observed on Venus) .
^^.fete^rf^^iifji i^t^^ianni gflrfirr ninf nti^r--- -ri r-- ^ "^-^fl-r^
Figure 10. The Earth at a distance of 70,000
km. Photograph obtained by the Soviet un-
manned spacecraft Zond-7.
With respect to the atmosphere of Mars, for some time we have been
confident of our knowledge, since this atmosphere is rarefied and we can
look right through it. It is true that until quite recently the role of
(2)
For greater detail, see Chapter 5.
30
light scattering by aerosols was underestimated, and we have overestimated
the amount of atmospheric pressure on the surface of Mars and assumed it to
be equal to 80-100 mbar, whereas its correct value is near 10 mbar (probably
less than more) . It is just such a value that is given by measurements of
different CO absorption bands, carried out on the Earth, and also Investiga- /33
tions of the damping of radio emission of the unmanned spacecraft Mariner (see
below, Section 5) in going behind the planet Mars. The difficulties arising in
interpreting the spectral and photometric measurements rest mainly on the
impossibility of taking into account the role of aerosols in the scattering
of light. It is quite probable that the atmosphere of Mars is significantly
colder than its surface. This is indicated by the estimate of temperature
from the structure of the atmospheric spectral bands on Mars and from the
results of radioscopy of the atmosphere by radio waves.
Cloud formations on Mars are an ordinary phenomenon, but quite variable.
Here dust storms and haze are observed. As a rule, the Martian atmosphere
strongly scatters violet rays, and when Mars is observed through blue or
violet filters, very little can be seen. But a blue clearing sometimes
exists in these rays also, and then the atmosphere of the planet is especially
transparent. In its random nature, such a transparency was characteristic
of the entire atmosphere of Mars at the time when Mariner-6 and Mariner-7,
flying near it, photographed its surface. Recently the "violet clouds" on
Mars have been found to be similar to terrestrial noctilucent clouds. But
we should not be deluded by this comparison. Its cognitive value is not very
great. It is sufficient to recall that arguments are still going on con-
cerning the nature of terrestrial noctilucent clouds.
As far as Jupiter's atmosphere is concerned, although for the past
several decades there has been some information about it, much remains
completely enigmatic. The bands visible on Jupiter, of course, are cloud
formations, the nature of which can be hypothetically established based on
theoretical arguments. The basic cloud layer consists probably of solid
particles of ammonium hydrosulfide (NH.HS) and small drops of an ammonia
solution in water (NH.OH) . The richness of the colors, mainly reds and
31
Figure 11. Photograph of Jupiter, taken
on February 16, 1968 through a green
light filter.
oranges, is created by cyanides of
HCN and C_N_, which by polymerizing
and converting to the solid state,
assume chemical stability. In the
laboratory these intensely colored
compounds are formed by electric
discharge in a mixture of methane
and ammonia (in the absence of
oxygen) . Higher up, above the
basic massive clouds, up to the
troposphere, crystalline particles
of solid ammonia are suspended
which form a light haze. The haze /34
is observed only by using specific
methods, and is found to be a
variable phenomenon. In particular,
Jupiter has its own type of "polar
caps" of these particles, whereas
Saturn has none.
Thus, in general, we can explain the diversity in colors and shades
visible on Jupiter. In particular, the dark bands of Jupiter are not gaps
in the cloud layer, but are simply clouds having a different composition.
All of these are comparatively high formations at the level of which the
pressure comprises several atmospheres (2-5 atm) , and the chemical composition
of the atmosphere as determined from spectral observations is the following:
(3)
methane (about 100 m-atm) , ammonia (on the order of 10 m-atm) , hydrogen /35
(85,000 m-atm). Based on theoretical arguments the atmosphere of Jupiter
should have an abundance of helium (about 26 km-atm) , but it is not amenable
to observations from Earth. These quantitative estimates are quite unreliable.
(3)
1 cm-atm characterizes the amount of gas in a column having a height
2
of 1 cm and an area of 1 cm at a pressure of 1 atm and a temperature of 0° C;
1 m-atm = 100 cm-atm; 1 km-atm = 1000 m-atm.
32
since the role of multiple scattering, which certainly is effective in the
atmosphere of Jupiter, is unknown. This is particularly indicated by the
fact that absorption in the methane bands is not amplified toward the edge
of Jupiter's disk, but rather is diminished. Therefore, the numbers given
above in meters-atmospheres refer to the gas content in the atmosphere of
Jupiter not only above the cloud layer, but inside this layer as well, along
a complicated and intricate path of light quanta which are multiply scattered
in the clouds.
Bearing in mind the low temperature of the upper atmosphere of Jupiter,
we have no right to expect indications of water vapors in its spectrum, but
as to whether they do, in fact, exist in the lower atmosphere, which is
warmer, is an important and interesting question, since this would incidentally
solve the problem of the existence of oxygen on Jupiter. Free oxygen or the
presence of a large amount of free hydrogen obviously does not exist. No less
interesting would be the establishment of carbon dioxide in the atmosphere
of Jupiter, which must also be destroyed by frost in the upper atmosphere.
Temperature measurements in the infrared rays gave values of 150 and 128°
K at wavelengths of A 20 y and X = 8-14 U, respectively. Radio measurements
in the range A < 3 cm gave approximately the same values in the range of
110-150° K. At the same time, the equilibrium temperature, that is, the
temperatures at which the heat received from the Sun and the heat emitted by
the planet into space are quantitatively equal, is 110° K for Jupiter —
that is, significantly less than the measurements give. Consequently, the
internal heat of the planet reaches the surface of Jupiter which we observed
and increases the heat flux leaving it by no lesd than 20%.
In measuring the emission from Jupiter in the radio band, there are
indications that the temperature increases with depth, and the variable
intensity of emission points to atmospheric activity. This is also in-
dicated, although not explained, by the appearance of "hot shadows" —
at a wavelength of X = 10 y an increase in temperature in the shadow cast,
on the planet by its satellites.
33
A decisive event in observational astronomy is the obscuration by a /36
planet during its travel across the sky of one of the stars of sufficiently
bright to be observed at the time of extinction as it passes beyond the
atmosphere of the planet approaching it. For the past ten years only three
such cases have been registered: obscuration by Venus of Regulus in 1959,
by Jupiter of a Aries in 1952 and by Neptune of a seventh magnitude star
BD-17°4388 in 1968. The photometric observations of the star during ob-
scuration make it possible to determine the altitude of the uniform atmosphere
(4)
H of the planet at various levels and then to find either molecular weight
or temperature, if the other quantity is known . If the temperature value of
100° K, which prevails in the upper layers of Jupiter's atmosphere Is proven,
for which H = 8.3 km has been found, then the molecular weight of its upper
atmosphere is near 3.8, which is similar to the molecular weight of helium
(4.0), which has not been spectroscoplcally detected in the atmospheres of
the planets. But hydrogen H„ (molecular weight p = 2) has been detected on
Jupiter in huge amounts, while the heavier CH, and NH , on the other hand, have
been found in relatively negligible amounts. Therefore, the molecular weight
]i = 3.8 indicates that, in terms of volume, helium makes up quite a signifi-
cant part of Jupiter's atmosphere; we mentioned this earlier. When our radar
equipment is found to be adequate for locating Jupiter's satellites, radioscopy
of its atmosphere will become a commonplace event, and will give much more
information concerning the scale of altitudes, horizontal movements in it,
and its dielectric properties.
Jupiter seems to be the most enigmatic of the planets. These enigmas
begin from its visible surface and extend both to the deepest and to the
uppermost layers, its exosphere.
The quantity H is also often called the "scale height", since it is
the difference in altitudes corresponding to the decrease in atmospheric
pressure by a factor of e (2.718...).
(5)
For greater detail, see Chapter 5.
34
The different angular rate of rotation of the various zones of Jupiter
depending on the their distance from the planet's equator (equatorial
acceleration) can be comprehended and probably theoretically Interpreted
based on the fact that the planet's atmosphere is very large, its rotation
is very rapid, and the centrifugal and Coriolis accelerations are high.
Spectral observations have revealed a strange incongruity between the linear /37
rotation velocity of the clouds and the gas component of Jupiter's atmosphere.
Sometimes it seems that the ammonia and the methane do not participate in the
planet's rotation; that is, the gas masses move counter to the rotation, which
obviously may be accomplished only at a level other than the level of the
clouds, and in any case, indicates a certain unusual type of atmospheric
circulation. But such an exceptionally stable formation as the Red Spot also
has its own special rotation period, so that this atmospheric formation moves
relative to the surrounding atmospheric masses, which in the vicinity of the
Red Spot have velocities up to 100 m/sec. There are indications that the
atmospheric circulation takes place around it with a period of 12 days.
This and many other factors indicate a tremendous atmospheric activity
on Jupiter, but the nature of the activity is not clear. The role of the
Sun in the thermal conditions of the planet even with a small penetration
into the atmosphere of Jupiter is small — the heat flux from the Sun there
is 27 times less than on Earth, and a significant part of it is reflected
into interplanetary space. The rapid rotation makes the time variations in
solar radiation insignificant during the day, and the small inclination of the
planet's axis and the almost circular orbit of Jupiter makes the variations
small during the year. We can understand the latitude distribution of the
atmospheric processes, because of the latitude dependence of solar radiation
and acceleration of gravity is constantly in operation on Jupiter . In
addition, to the latitudinal heterogeneities there are considerable longitudi-
nal heterogeneities.
Deeper into the atmosphere of Jupiter, we encounter ever-increasing
temperature and pressure. This latter may reach values at which H and He
35
are converted into the solid state; at various latitudes, such a conversion
will take place at different depths, and there must not he any longitudinal
variation here. Thus, the question arises as to whether those layers of the
planet are chemically and physically uniform where the atmosphere loses its
meaning .
The theory which suggests a uniform composition, of course, does not
agree with this concept. The mass, radius, and moment of inertia of Jupiter
(and Saturn) indicate a low density of the planetary matter, that is, either /38
a predominantly hydrogen composition (with helium admixture) or a high
temperature in its interior, so that the elasticity of the gases successfully
resists hydrostatic pressure. Here considerable convection may take place as
well as an energy transport of heat to the outside, which is confirmed by
observation only to a small degree.
Another group of incomprehensible phenomena on Jupiter involves, on the
other hand, its surface regions. As was said above, the brightness temperature
of Jupiter at wavelengths of A < 3 cm corresponds to the temperature measured
in the infrared region. Already at a wavelength of A = 10.3 cm the emission
from Jupiter corresponds to a temperature of about 600° K; at A = 22 cm,
3000° K; at A = 31 cm, 5500° K; and at A = 68 cm, 70,000° K; that is, these
emissions are clearly of nonthermal origin and indicate the existence near
Jupiter of a powerful magnetic field (many times stronger than the magnetic
field of the Earth) and a belt of high-energy particles, similar to the
radiation belt of the Earth. These particles move in the trap of the magnetic
field and are de-excited by the mechanism of synchrotron radiation. The
decimeter radio emission from Jupiter is partially polarized, which makes
it possible to establish the direction of the planet's magnetic axis, that is
Inclined from the axis of rotation by 10° . The polarization plane varies
slightly with the period, equal to the period of rotation of Jupiter, and
this reveals a misalignment of the axes.
As mentioned earlier, the rotation period of Jupiter is not uniform for
objects of the equatorial and the middle zones. For these, we must introduce
36
two systems of computing the longitudes: System I with a rotation period of
9^50™30^. 003 and System II with a period of 9^55™40^.632. The fluctuations
in the polarization plane of the decimeter emission from Jupiter take place
at a period of 9 55 29 .37, that is, 11 seconds shorter than the rotation
period of System II, which pertains to the middle latitudes. System III for
computing the longitudes is determined in this way.
The dimensions of the region of radio emission from Jupiter at decimeter
wavelengths exceed its optical dimensions by far, reaching 2-3 planetary
radii at the equatorial zone, although traces of this radiation may be noted
also at a distance up to six radii. It is mainly this picture which serves
as the basis for explaining the nonthermal emission from Jupiter by processes
in the radiation belt. In the decimeter band, Jupiter is one of the most
powerful sources of radio emission in the sky.
But in the decameter band (X > 7m), Jupiter gives powerful bursts /39
originating from discrete sources, also rotating with the period of System III .
These bursts, 1-2 sec in duration or very short and on the order of 0.3 sec,
have been known to radio physicists for some time, but only in 1955 were they
linked to Jupiter. Even the Sun does not give such powerful impulses at
decameter wavelengths. The dimensions of the sources, established inter-
ferometrically, are 10-15 or 30-40,000 km, but apparently this value is
strongly exaggerated by the scattering of radio waves in interplanetary space.
It is remarkable that the decameter radiation from Jupiter depends on the
position of the planet's magnetic axis relative to the observer from Earth;
it is strongest when the northern magnetic band passes through the central
meridian. It is even more remarkable that this radiation depends on the
position of the nearest of the Galilean satellites of Jupiter — lo : its
Intensity is maximal when the longitude of lo, computed from the superior
geocentric conjunction, is equal to 90° or 240°. Jupiter's magnetosphere
extends up to lo's orbit. Does lo affect the magnetosphere by a hypothetical
magnetic tail or is it purely gravitational? This remains unclear. The
source of decameter emission probably is different from the decimeter
37
emission. Just as in the case of bursts of radio emission from the Sun,
the cause may lie in the plasma oscillations. But this is only a hypothesis.
Thus, the radio emission, originating from the outermost layers of
Jupiter's atmosphere, does not depend on the rotation of the visible cloud
surface of the planet. But it is created in the magnetosphere , which is
determined by the material carrier of the planet's magnetic field, connected
with the body of the planet and with its inner regions. Perhaps the rotation
period of System III is the true rotation period of the planet, and the
rotation of Systems I and II reflects certain systematic motions of the gas
mantle of Jupiter: for latitudes greater than 12°, at a velocity of 4 m/sec,
and at the equator, up to 110 m/sec.
After Jupiter, Saturn's atmosphere presents nothing new. The chemical
composition is the same, only the presence of ammonia is uncertain. This is /40
easy to understand since ammonia must be in the solid state at the lower
temperature of Saturn. The temperatures of Saturn measured in the infrared
band vary in a range from 85 to 125° K. The lower values are preferred,
since they are confirmed by radio measurements: 97° K at A = 3.2 mm, and
from 96 to 116° K at A = 8.6 mm. With increase in wavelength, the temperature
grows; 190° K at X = 11.3 cm and about 300° K at A = 21.3 cm. This indicates
a slow elevation in temperature with depth: short-wave radiation does not
reach us from the greater depths. Here there is no similarity to the sharp
increase in brightness temperature which was mentioned for Jupiter and
interpreted by us as sjmchotron radiation in the magnetosphere. We know of
no indications of a magnetic field on Saturn (polarization of the radio
emission is not certain) .
On the disk of Saturn the details observed are much smaller than those /41
on Jupiter, probably because the clouds of Saturn consist of methane rather
than of ammonia. Saturn's atmosphere is more extensive than that of Jupiter,
and the differentiation in rotation velocity with latitude is stronger
(lo'^14"' at the equator, and 10^^40™ at a latitude of 50°).
38
Figure 12. Photograph of Saturn and its rings.
Uranus and Neptune
continue the trend mentioned
in the transition from
Jupiter to Saturn: intensi-
fication of the absorption
bands of methane and hydrogen.
But the existing quantitative
estimates of the amount of
methane (150 km-atm on Uranus,
and 250 km-atm on Neptune)
are highly unreliable and may
be in error by an order of
magnitude. The brightness
temperature of Uranus was
measured with a very high
error: T = 100° + 35° K for A = 6 cm and T = 128° + 40° K for A = 11.3 cm.
We do not know to which level this pertains. If the heating were only from
the Sun, the equilibrium temperature would be only 60° K. It may be, just as
on Jupiter, that a notable heat flux comes from the interior of the planet.
Uranus and Neptune possess the highest reflectivity of any of the planets of
the solar system (albedo of 0.93 and 0.84, respectively). The brightness
temperature of Neptune is found from radio observations to equal 180° + 40° K
at A = 1.2 cm and 115° + 36° K at A = 3.12 cm. Photometric observations
show a temperature of 110°-130° K for sufficiently high layers of the
atmosphere, and a very slow drop in density with altitude; the altitude of
the uniform atmosphere is H = 50 km, which may be explained by the rich
amount of hydrogen.
Pluto, the last member of our planetary system, has remained completely
unstudied, but it is in no way similar to the giant planets; even its
rotation is slow, with a period of 6.39 days. Its dimensions are known
with a low degree of reliability, and, therefore, the low value of the
albedo (0.14) derived from its visible brightness is also uncertain. If we
accept this, then we must assume that Pluto's atmosphere is insignificant.
39
We know of 32 planetary satellites in the solar system. Their dimensions
are quite different — from several kilometers (Deimos, satellite of Mars)
to several thousands of kilometers (the Moon, the Galilean satellites of
Jupiter, Titan — satellite of Saturn, Triton — satellite of Neptune), but
only for Titan has an atmosphere of methane been reliably observed.
40
^' INTERNAL STRUCTURE OF THE PLANETS
The internal structure of the planets cannot be a topic for direct ob- /42
servations. Only certain integral characteristics of a planet are functions
of its internal structure, but the functional dependence is not unique, so
that the investigator can only construct a guess as to a planet's structure,
without pretending to have accurate knowledge. Knowledge of the temperature,
density, chemical composition, and existence of phase modifications of matter
as a function of depth could give a great deal of information for solving the
problem of a planet's formation, be it cooling of the primitive mass ejected
from the Sun or accretion by the planetary nucleus of the matter surrounding
it, or condensation of matter in a constringent gas-dust cloud in the presence
of the Sun. The preference for one of these three possibilities would open
the way to solving still another question — the frequency of the process of
forming planetary systems in the Galaxy. In addition, the question of the
primary chemical composition of matter producing the solar system would be
explained.
Unfortunately, we are still quite far from answering these questions.
Here we can see the vast scope of theory, but not experiments.
In addition to mass, radius, and mean density following from it, we
still know only the moment of inertia of the planet among the integral
characteristics. The surface temperature in no way determines the distribu-
tion of temperature in the depths. The existence or the absence of a
magnetic field on a planet would give certain indications on the internal
structure of a planet if we had a reliable theory for the onset of a planet's
magnetic field, even of our own Earth.
Even living on Earth, we know little about its internal structure.
Concepts exist in geophysics about this that are mutually contradictory and
estimates of the mean temperature differ by factors of two-three. Abrupt
changes in density at the boundaries of various zones inside the Earth,
41
established by seismic observations, are Interpreted by some authors as
signs of a varying chemical composition and by others as the result of
change in the phase state of matter.
But if we do not attempt to explain the details, knowledge of the mean /43
density of a planet as a whole — that is, the arithmetic result of dividing
the planetary mass by its volume — results in a topic for discussion. Thus,
for example, planets of the Earth group located inside the ring of asteroids,
that is. Mercury, Venus, Earth, and Mars, possess a high mean density of 4-6
3
g/cm , whereas the giant planets, Jupiter, Saturn, Uranus, and Neptune, have
a density significantly smaller. Jupiter and Saturn have a mean density
less than the gaseous Sun. The mean density of Saturn is half that of the
Sun.
One assumption stipulates that there is a substantially different
chemical composition and different types of structures for certain groups of
planets. The inner planets have a higher density which is naturally
attributed to the presence in their interiors of iron, a heavy element
which is widely distributed throughout the universe. The lower density of
the giant planets can be understood if we assume their chemical composition
to be near that of the Sun and the stars, where the lightest elements,
hydrogen and helium, predominate.
Prior to 1920, it was assumed that Jupiter and Saturn are uncooled
planets because of their low density, fast surface variability, and the
existence on Jupiter of the Red Spot. They were considered as a peculiar
type of small sun, not so hot as the Sun, but nevertheless, in the "fire-
liquid" phase. Therefore, publication of the results of the first measurements
of the thermal fluxes from these planets, indicating a very low temperature,
came as a shock. Theory helped recover from the shock. Namely, the theore-
tical discussions did not include such small (in comparison with the Sun)
bodies in the self-luminous category, since at a high surface temperature
they would have to "burn up" their small reserves of heat in a short period.
Then they were assumed to all be completely cold bodies. This also was
42
inaccurate. Let us recall that the temperature measurements of Venus in
the infrared band gave a value of 240° K, and in the decimeter wavelengths
it indicates a temperature of up to 700° K. The first value refers to the
upper atmosphere, the upper boundary of the clouds, and the second to the
planet's surface. Between one and the other level, the temperature drops in
a regular fashion as a result of the fact that absorption of its thermal /44
emission takes place in the planet's atmosphere. In the cloud layer there
is also a strong scattering. The atmosphere itself is sufficiently extensive to
accommodate the processes of damping of the outgoing radiation.
The same factors must operate also in the huge atmospheres of the outer
planets, Jupiter, Saturn, Uranus, and Neptune, so that in their depths the
temperatures must reach thousands of degrees. We have seen signs of internal
heat outflow from these planets, first of all in that the temperature
measured in them is higher than equilibrium temperature, and secondly, during
the measurements at the longer waves of the radio band the observed tempera-
ture was higher.
Another integral characteristic of the planet, its moment of inertia,
is determined from the motion of the line of nodes or the line of apsides
of the orbits of the satellites or from the flattening of the planet if the
planet is in hydrostatic equilibrium. The theory has been well developed
only for slow rotation. The ratio of the moment of inertia I to the moment
of inertia ^/a 3Ri?' of an equally large globe, the entire mass of which is
distributed along the surface, is equal to 3/5 for a uniform sphere, and to
zero for a body with a mass concentrated at the center. On Earth and Mars,
the ratio I'.^^U^R^) is equal to 0,50 and 0.58, respectively, thus indi-
cating a sufficiently high uniformity, and for the outer planets it is shifted
The line of nodes is the line of intersection of the orbital plane
of the satellite with the orbital plane of the planet itself around the Sun
or (in the case of artificial satellites) with the equatorial plane of the
planet. The line of apsides is the major axis of the orbital ellipse, con-
necting the nearest and the farthest position of the satellite relative to
the planet.
43
toward 0.39-0.31, that is, toward a greater heterogeneity. The theory which
takes into account the rapid rotation of Jupiter and Saturn indicates large
mean densities of 2.7 and 1.7, respectively. The purely hydrogen-helium
composition of these planets would not be allowable if it had not been for
the discovery (theoretically) of a metallic modification of hydrogen at a
pressure of 5,000,000 atm. At 30,000,000 atmospheres, the density of
3 3
hydrogen equals 3.1 g/cm , and of helium — 7.6 g/cm .
For these two planets, acceptable models are found with a relative
amount of hydrogen of 80 and 68%. The models which agree with the observed /45
values of I have been found for Jupiter and Saturn also, with a hydrogen
content of 78 and 63%, respectively. At the center of these planets, where
3 3
the theoretically computed density reaches 31 g/cm and 16 g/cm , helium
sharply predominates.
We have cited these low-reliability numerical characteristics in order
that we might give some idea as to what the internal structure of the giant
planets may be ; this is derived on the basis of a comparatively simple
theory. Even for Uranus and Neptune, which have a comparatively high mean
density of 1.47 and 1.88, respectively (versus 1.30 and 0.71 for Jupiter
and Saturn), with relatively small dimensions, the hydrogen-helium composition
does not fit. We must introduce into the examination the ice of water,
methane, ammonia, hydrogen sulfide, oxides of metals and even metals. But
by varying their content, we can obtain all the integral characteristics.
It would be very important to know how far the atmosphere of Jupiter
and Saturn extends, if a liquid layer exists at the bottom of the atmosphere,
or if the atmosphere and the solid surface come into contact. The theory
assumes that both atmospheres are quite extensive, and comprise, respectively,
20 and 50% of the mass of the entire planet. Then at their bases there must
be such high pressures that a liquid phase is Impossible.
Furthermore, as we have seen in the previous chapter, Jupiter and, to
a lesser degree, Saturn, have large heterogeneities under the cloud layer;
44
otherwise we would not observe the diversity in forms of the cloud surface
of these planets. We can, therefore, pose the question as to the internal
activity of Jupiter and Saturn, similar to the manner in which we pose the
question of solar activity.
The inner planets, of course, are more complex to comprehend. Although
it is easier to make an analogy with Earth here, the indeterminancy of the
answer is not diminished, and, in particular, it remains debatable as to
whether one or another planet possesses a core. One of the most popular
theories of the Earth's magnetism relates the existence of a magnetic field
with the dynamo mechanism in a liquid conducting core where convection takes
place. At the present time, the direct contacts of the unmanned space
stations with the Moon, Venus, and Mars have shown that none of these
celestial bodies has any notable magnetic field.
The existence of a radiation belt on Jupiter proves the existence of a 746
magnetic field on it. As we have seen above, Saturn has no clear indications
of a radiation belt. Nothing is known about this on Uranus and Neptune.
Thus, of all the planets of the solar system, only on Earth and Jupiter can
we state with confidence that a magnetic field exists and possibly only on
these two planets are there liquid cores. We cannot exclude the possibility
that Jupiter has a magnetic, productive core which reaches the atmosphere
and that certain of its atmospheric formations, such as the Red Spot, have
a connection with the magnetosphere.
The difficulties entailed in a theoretical investigation of the internal
structure of the planets follow not only from the poor knowledge (or lack of
knowledge) of the phase states of the various materials at high pressures or
temperatures of several thousands of degrees — which are, apparently,
typical for the interiors of the planets — but also from the incompleteness
of our concepts concerning the transport of heat inside the planet. Thus,
assuming that in the formation of the planet radioactive decay heated the
interior of Jupiter, we can find a temperature differential of 10,000°
between its center and surface, if the mechanism of thermal conductivity
45
operates. It is much smaller if, to the thermal conductivity, we add
transport by conduction and radiation. But the theory of radiation trans-
port inside a planet has not been completely developed.
The appropriateness of one or another model of the internal structure
of a planet is examined not only from its integral characteristics, but also
from the agreement of the model with our concepts concerning the past
history of the planet, and this in turn depends on the way in which the
solar system was formed. At the present time, it is most probable that the
planets were formed by condensation of matter from a gas-dust cloud,
separately into a central star and separately into planets. Before the
large planets were formed, small bodies — planetesimals — were formed,
which were then combined into larger ones. Here the kinetic energy, due to
inelastic collisions was converted into thermal energy. The newly formed
planet was warmed up. Radioactive decay was another source of internal heat,
which was no less if no more effective. The planetary matter was converted
from the crystal state either into a melted state if the high pressure did not
prevent this or, remaining solid, changed into another modification. At /47
sufficient pressure, it was metallized, that is, under the influence of
pressure the bound electrons of the atoms and molecules passed into the zone
of conductivity, and this substantially increased the thermal conductivity
of the matter. But increase in thermal conductivity increases the transport
of heat from the depths of the planet to the outside. If these materials,
for example, silicates, are not metallized and remain crystalline, then
their thermal conductivity drops with elevation in temperature, thus facili-
tating heating of the planet. In addition to the conductivity, the heat is
transported by the silicates via radiative transport.
As we see, the picture is rather complex, and if we do not wish to end
in a controversy, all the above processes must agree with our concepts on
the age of the solar system. Judging from the age of the Earth, the
formation of the solar system took place about six billion years ago and
already during the first 200 million years the planets were heated approxi-
mately as in our time. Their further thermal history is determined by the
46
radioactive decay of their matter. The mass of the planet determines the
content of radioactive materials in absolute numbers.
On Earth only silicate rocks possess radioactivity; they are more
abundant in the crust than in the mantle. Rocks containing iron are free
of radioactivity. In the giant planets with their overwhelming amount of
light elements, the radioactivity is weak, but the reserves of heat,
accumulated in the formation of the planet (due to the energy potential) are
so high that for the entire time of their existence — for example, Jupiter —
the internal temperature has been lowered by no more than 1000° K.
We might think that the planets of the Earth group have an internal
structure similar to that of the Earth. We assume its crust has a thickness
3
of 18-20 km with a mean density of about 2.5 g/cm . Beneath it is the
mantle, in which the density increases with depth, first rapidly and then
slowly. The core is even deeper.
The physical heterogeneity of the Earth's structure is indicated by its
elastic properties as they appear from the propagation of seismic waves.
At a depth of more than 2900 km, no transverse elastic oscillations are
propagated, and a sudden change takes place in the properties with increase
in density. This change can be attributed to change in the chemical composi- 748
tion, for example, to the fact that the heavier elements are ejected through
the viscous magma of the mantle nearer the center where they accumulate,
forming a heavy core. But it is possible, using the concept of the uniformity
of the Earth's chemical composition, to explain the abrupt change in elastic
properties and density by the conversion of olivine rocks (a mixture of mag-
nesium and iron orthosilicates) under the influence of high pressure from
the ordinary crystalline phase state into a metallized state. This transition
changes the olivine into a liquid state of high density, characteristic of the
Earth's core, in which about one third of the Earth's mass is contained.
But at the very center there is still another core in which about 8% of the
core's mass is located, or altogether only 2% of the Earth's mass. This
core consists of iron and nickel. It is solid, unlike the larger core.
47
Thus, the Earth, used as an example, has a multilayer structure, which may
or may not be applicable for explaining the structure of the other planets
of the Earth group.
Mercury, with its small mass, has the greatest density in the solar
system. In it is either a small iron core, surrounded by silicates with an
iron inclusion, or iron and nickel are distributed evBryvrh.e.re with the
silicates. But they are not molten due to the sparsity of radioactive
elements. For the last two billion years. Mercury has been cooling off.
Venus, which is similar to the Earth in mass and dimensions, certainly
has an iron core and a core of metallized silicates. The latter contains
about one fourth of the entire mass of the planet. At its boundary a
pressure of 1.5 million atmospheres is reached, which makes metallization
possible. The core may be molten, but a crust is located only at the surface.
Finally, Mars, probably has a small iron core (7% of the mass) and a
very thin crust.
The specific difficulty in constructing models of the inner planets
lies in the impossibility of controlling the (other than the Earth) magnitude
of their moment of inertia. For Venus and Mercury, which have no satellites
and with practically a spherical shape, which is natural with a very slow
rotation, the moment of inertia cannot be determined. A flattening is
observed on Mars which significantly exceeds the theoretically expected value /49
from the most widely accepted assumptions on the distribution of masses. It
may be that the observations of the shape of Mars' disk involve some systematic
error. In passing, we should mention that the flattening of the Earth also
is greater than that which would be expected from its rotation velocity, if
our planet were in hydrostatic equilibrium.
Directly related to this subject of the internal structure of the planets
of the Earth group is the question of the formation of their surfaces. We
have already mentioned this at the beginning, in connection with the
48
Figure 13. Rills on the Moon. The so-
called "Cobra's Head" in Schroter's
Valley. The picture was taken by the
apparatus on the Lunar Orbiter. Frame
size 4 X 4 km.
seas appear as the result of lava eruptions on
for assuming molten rock on the Moon, tectonic
processes in the crust, and volcanic processes
description of the lunar and
Martian landscapes; only for
these two objects is the land-
scape known to us sufficiently
well. We have also turned our
attention to the fact that in
the formation of the lunar
surface both internal and
external factors played a role.
If in the majority of cases it
is necessary to assume an ex-
ternal impact of large outside
masses for the formation of the /50
circular mountains , then for the
formation of mountain ranges
on the moon, tectonic processes
are essential that are asso-
ciated with the elastic tension
in the crust, and the lunar
a huge scale. Is there a basis
movements based on the thermal
of small and large scale?
This last question sounds somewhat rhetorical after N. A. Kozyrev
observed, if not direct volcanic damage, then an abundant generation of
gases accompanying the damage inside the lunar crater Alphonsus. But what
does the theory of the internal structure of the planets say on this score?
We know the mass, radius and moments of inertia of the Moon with respect
to the different axes, sufficiently well to construct a model of the Moon
with confidence. The lunar mass is extremely small, and, therefore, the
pressure at its depths nowhere reaches such values that metallization of the
silicates could take place, so that the Moon has no core. The lunar mantle
contains a sufficient amount of radioactive elements which heat it up before
49
melting, which takes place at a depth of about 300-400 km. In any case radio
measurements at various wavelengths of the heat flux leaving the Moon clearly
indicate a rather rapid increase in temperature with depth, caused probably
by a high concentration (four times higher than on Earth) of radioactive
elements near the lunar surface.
Individual sites of the lunar surface are found to be much hotter than
their surroundings. For example, such are the numerous craters and cirques of
large and small dimensions such as Tycho, Copernicus, Kepler, Hosting C, and
several of the seas such as Mare Tranquillitatis, Mare Serenitatis,Mare Humor-
um, which appear especially in relief under infrared observations during total
lunar eclipses. These "hot points" can be easily explained by the fact that
they are composed of rocks with high thermal conductivity, which easily
transport the internal heat to the lunar surface. But this may also be heat
which is accumulated by the upper surface layers during the long lunar day.
On the other hand, recently a formation was detected which extended into a
long band along the western boundary of Mare Humorum (toward the south from
the crater Gassendl) , which is constantly hotter than the surrounding sites
outside the eclipse, at the height of the lunar day. Here we encounter a /51
nonequilibrium process, the cause of which is the real transport of heat along
paths created by the structure of the lunar crust at a given site (fractures,
faults. . .) .
Thus, theory and observation fully indicate the existence on the Moon
of high-temperature zones capable of producing tectonic processes and vol-
canic phenomena, and in the same manner help us understand the processes
which take place on the surface of the Moon and to understand how the various
details of the lunar landscape were formed. An interesting and important
discovery in this respect in recent years was the discovery beneath the lunar
seas of heavy masses, called mass-concentrations. The mass-concentrations
were found to be anomalies in the motion of the artificial lunar satellites.
They are rather numerous, but exist only beneath the seas, having a regular
shape. We might think that these are residues of especially large planetesi-
mals which pierced the crust of the Moon when they fell and produced vast
50
lava eruptions. The masses of the mass-concentrations comprise 10 -10 of
the lunar mass.
Other explanations also exist for the mass-concentrations, for example,
as formations of hardened lava of high density which, after impact of the
planetesimals, were extruded upward and formed huge formations, heavier than
the surrounding continental rocks. Due to its large specific weight, such a
formation even with a smaller expanse in depth is capable of rendering the
same pressure on the upper boundary of the plastic mantle as the more
extensive, but lighter continental and subcontinental formations. (Herein,
as we know, lies the hypothesis of Isostasy first expressed with respect to
the Earth more than 100 years ago) . In this explanation there is no need to
attribute to the impacted planetesimal an excessively high density, which is
only slightly probable since planetesimals of iron-nickel composition could
hardly ever have existed. No matter what the case may be, the existence of
large heavy heterogeneities under the surface of the Moon indicates that the
Moon possesses a sufficiently thick crust above the magnetic mantle.
The completely external formation — the rings of Saturn — we shall
examine in the chapter on the internal structure of planets, because it has
nothing in common in its nature with the surface of Saturn or its atmosphere. /52
On the contrary, it may be explained as a relict phenomenon, indicating the
initial conditions which accompanied the formation of the planets four to
six billions years ago.
From the time, more than 100 years ago, when Maxwell theoretically
showed that the rings of Saturn cannot be integral, solid formations, and
Belopol'skiy proved this experimentally by spectral observations, there has
been no lack of explanations for the nature of the rings, mainly on the
basis of their photometric study, during a change in the position of the
rings relative to the Earth and the Sun. It was established that in the
reflection of solar light the most important role is played by the mutual
eclipse of the Individual blocks comprising the rings. But what are the
dimensions of these blocks? Unfortunately, the opinions of theorists
51
disagreed, and even now some cite proofs that the rings consist of fine
particles with dimensions in microns, whereas others speak of a conglomerate
of blocks, up to two meters in cross section and smaller fragments (on the
order of centimeters and less). Probably the truth lies with the latter.
As far as the thickness of the rings is concerned, it can hardly exceed 3 km,
since from the myriad of "satellites" of Saturn forming the rings, only those
have been left intact which moved in the equatorial plane of the planet.
It is very difficult to establish the chemical composition of the blocks.
The spectrum of the rings in the infrared band reveals, as in the Martian
polar cap, absorption bands that are characteristic of ice or hoarfrost. As
to whether ice is found only on the surface of the blocks, or the blocks as
a whole consist of ice, as yet is unknown.
52
5. INVESTIGATION PROCEDURES AND POINTS OF APPLICATION
Radar observation of the planets . Among the investigation methods and /53
their practical utilization, the richest possibilities are afforded by radar
if it can be used effectively with respect to Jupiter and at the distance of
this planet have a resolving power of even 1000 km, which corresponds to about
1/3 second of arc. At the decameter wavelengths, a mirror (or complex group
of components) would be required for this with a diameter greater than
10,000 kml In the centimeter band the necessary dimensions of the mirror
3
would be 10 times smaller, that is, 10 km (at a wavelength of A = 1.7 cm).
The effective wavelength might be decreased another factor of 10, and then
the dimensions of the mirror would become realistic. But the problem of
radar in the future will consist of analyzing the planetary surfaces with the
aid of radio waves where optical means cannot penetrate the atmosphere of a
planet and its clouds. For this, the millimeter waves are not suitable,
since they are absorbed in the majority of planetary atmospheres and es-
pecially in water vapors. The decimeter waves apparently encounter a barrier
In the ionospheres of the planets if these latter are sufficiently dense and
it is these which must undoubtedly be used for scanning Jupiter.
Fortunately, contemporary powerful computer technology permits increasing
the resolving power of our telescopes without resorting to huge mirrors, but
by using an ingenious combination of mirrors of smaller dimensions and
mathematical analysis of the incoming radio signal. It is true that the
quality of the impulse returning after reflection is decreased (signal to
noise ratio) but the information obtained is, nevertheless, quite substantial.
An example is the reproduction shown on Figure 8 of the radar photograph of
the region of the region of the Moon around the crater Tycho. It was obtained
with the aid of a mirror having a diameter of 37 m at a wavelength of 3.8 cm,
by Lincoln Laboratory in the USA. It shows the reflectivity of the lunar
surface with a resolution of about 1 km. In order to obtain a resolution
with one antenna, a mirror with a diameter of 18 km would be required I
53
The radar method of studying the
surface of planets and their rotation
takes advantage of the fact that the
impulse reflected from the planet
carries in itself information of three /54
types: geometric (concerning distance),
kinematic (concerning the approach or
recession velocity) , and physical
(concerning the reflectivity of the
Figure 14. Schematic of radar probe site of reflection) . The first type
of a planet (see text). . .^ ^ . , . ^ . ,
xs manxfested xn the txme of arrxval
of the reflected signal, the second —
in the frequency of reflection of the signal, or more precisely, by the
frequency shift relative to the transmitted signal, and the third — in the
strength of the returned signal. Turning to Figure 14, we can see that
circles on the sphere of the planet, having as a common center that point of
the planet (subradar point) , for which the Earth is located at zenith, are
the geometric locus of the same lag in the reflected impulse. Let us select
the plane XOZ, comprising the axis of rotation of the planet OX and the
direction to the Earth OZ. Then the intersection of the surface of the planet
with the plane parallel to the plane XOZ will be the geometric locus of the
points having the same projection of velocity along the line of sight OZ
during rotation of the planet. According to the Doppler-Flzeau principle, it
gives the same frequency shift of the reflected signal with respect to the
frequency of the signal sent. If we expand the reflected signal according to
frequency (under the condition that the transmitted Impulse is strictly
monochromatic) , then this will be equivalent to scanning the disk of the
planet with a narrow slit. The Intensity of the signal with a given deviation
Av from the frequency v of the transmitted signal characterizes the amount /55
of energy reflected in a given band expressed on the disk by the "Interval of
frequencies" Av , Av , . . . (see Figure 14). With a combined analysis of the
frequency shift and the lag time of the reflected signal, we can even localize
those sites on the planet's disk where an increased or diminished intensity
54
of reflection is observed, although it is true in the general case that the
solution is ambiguous.
At first glance, it seems that in the kinematic phenomena observed by
the Doppler shift, nothing changes if the figure of the planet rotates with
its axis of rotation around the line of sight on Figure 14. Such, in fact,
is the case in observing the rotation of stars. We cannot determine the
position of the axis of rotation of a star on the plane of a figure, per-
pendicular to the line of sight, because the position of the terrestrial
observer relative to the star remains practically constant (in the framework
of the annual parallax of the star). A planet is another matter. The
terrestrial observer with his radar equipment continuously changes his
position relative to the axis of rotation of the planet, and the observed
rotation is the sum of the axial and orbital rotations of the Earth and the
planet. All these motions are known in advance, other than the axial
rotation, and may be taken into account in advance. But since their sum
vector constantly changes its position in space relative to the vector of
axial rotation of the planet, the position of the latter may be derived from
observations if they are continued for a sufficiently long period of time.
The next example, pertaining to astronomers, will explain the matter.
It is known that in opposition the upper planet moves with a retrograde
motion most rapidly. Thus, Mars, observed from the Earth, moves during this
time on a background of stars in the direction of diurnal rotation of the
celestial arc. For the observer on Mars, during this time. Earth would seem
to be rotating the most rapidly, and if the direction of the Earth's rotation
were retrograde, during the time of the opposition of Mars, it would appear
to the Martian observer to be the slowest. But this situation is fully
reproducible also for the Earth observer when he observes Venus during the
time of its inferior conjunction. Radar observations have shown that the
width of the signal reflected by Venus is lowest in frequency rather than
greatest at the moment of inferior conjunction. Hence, it follows that the /56
rotation of Venus is retrograde, and the width of the signal gives a linear
55
II I iiniiBiiiiiiii I Hill I
7.II.W6B
C'EOO
Z9.ri.WBi\
w
X
Figure 15. Comparison of profiles of
radar signals reflected from Venus
at a wavelength of 39 cm in two
inferior conjunctions of the planet
in January-February 1966 and in
June 1964. The repetition of de-
tails with respect to their
reflectivity can be seen.
rotation velocity for Venus at the
equator, whence the period of rotation
is derived. Finally, the law governing
the change in the signal width with
time establishes the orientation of
the axis of rotation in space.
The rotation period of Venus is
most precisely derived by comparing
the relationship of the frequency
profiles of the reflected signals
during the times of the different
inferior conjunctions. One or another
detail of this profile is repeated
during the time of subsequent conjunc-
tions. Its appearance at one and the
same place of the profile indicates
that a complete number of synodic
(that is, relative to the Earth)
periods of rotation of Venus has taken
place. The transition from the synodic period S to the stellar period P is
given by the formula cited (for circular orbits) by Copernicus : 151
B ^ P S '
where E is the period of rotation of the Earth around the Sun. If the rotation
of Venus were forward, then a minus sign would be placed in front of 1/P. It
is natural that S must be taken in absolute value.
The signal transmitted by radar has circular polarization. After specular
reflection from sufficiently large details on the surface of a planet, it
returns to Earth still polarized in the same manner, but opposed to the
direction of rotation. A sufficiently rough landscape makes possible a
specular reflection even from those places on the planet's disk which are
56
distant from the center all the way up to the limb Itself. On the other
hand, the planet with a smooth surface gives a polarized reflection only
from the central parts of the disk. Its effective cross section is greatly
reduced, and for radar probes of the planet a more powerful impulse must
be used.
If the reflecting surface has numerous irregularities, the radius of
curvature of which is less than the wavelength of the incident signal,
polarizations in the inverse direction do not set in, and the reflected
signal retains the direction of the circular polarization of the Incident
signal. This part of the reflected signal is called its depolarized com-
ponent, which may be the subject of a special Investigation when the signal
arrives at the receiving antenna. In summation, radar permits investigating
separately the polarized and unpolarized components of the reflected radia-
tion, and from this the structure of the planet's surface can be judged.
The coefficient of reflection during normal incidence makes it possible to
find the dielectric constant of the surface materials, that is, their
physical characteristics.
Let us note finally that the possible rotation of the plane of polari-
zation during the propagation of radar Impulses permits investigating the
electrical state of the Interplanetary and circumplanetary plasma, and also
the magnetic field around the planet on the basis of the Faraday effect.
Thus, the greatest information Is contained In radar-reflected signals,
which allows us to discover many facts that had previously been inaccessible.
Earlier we mentioned a number of such facts about Venus, which is covered /58
with a dense cloud layer. The large amount of Information reflects the
scope of the investigation method.
Radar gives the hope of collecting the most valuable information on the
surface of Jupiter, to establish whether the surface adjacent to the at-
mosphere is solid or liquid, to find the period of rotation of this surface,
the degree of its geometric and physical heterogeneity, the relationship
57
between the surface and cloud formations, and many other facts which have
been difficult to predict.
But we encounter one basic difficulty in this approach, that is, the
inadequate strength of the signal and the insufficient area of the antenna
for sending and receiving the signal in the case of such a remote object
as Jupiter, or such a small one as Mercury. The strength of the reflected
signal, received by the antenna, is inversely proportional to the fourth
power of the distance to the planet, and directly proportional only to the
first power of the strength of the transmitted signal. The signal is
damped in proportion to the square of the distance in the outgoing and
incoming leg. Although, when the irradiator is placed at the focal point
of the parabolic mirror, the beam of radio waves transmitted by the antenna
must be parallel, diffraction makes it divergent within the limits of the
angle of the directional diagram, which is always found to be greater than
the angular diameter of the planet's disk. If the cross section increases
In proportion to the square of the distance and becomes greater than its
dimensions near the planet, then part of it will be wasted for the experiment.
Therefore, the effectiveness of the radar experiment is higher, the narrower
the directional diagram, and, therefore, is proportional to the square of
the mirror's diameter. But this same mirror is in operation during the
reception of the reflected signal, so that the success of the radar probe
of the planets is defined as the fourth power of the diameter of the mirror.
From Figure 16, which shows how many times the strength of the signal is
attenuated travelling back and forth, it is clear that even the radar probe
of Jupiter, if we expect reliable results from it, will require amplifying
the strength of the transmitted impulse and increasing the dimensions of the
antenna. In fact, at the present time the only fully reliable data, except
for the Moon, are those of the inner planets, mainly Venus, and with the
greatest success in the upper Interval indicated on Figure 16. The diffi-
culties of radar scanning of Mercury have also been successfully overcome. /59
For the last two oppositions. Mars has also become an obedient subject for
radar Investigations. Jupiter requires improvement and intensification of
technology. Although the data concerning Its radar scanning have been
58
■iniiiiiiiiiiiiiiii
m
300
810
'^KO
^sso
(U
o
•a 350
—r\ — I — I I I Hit
-1 1 I I I M I
o[Sun]
Mar
(June
(December
CO
I 380
10^
f^[ Jupiter] -
6 Icarus .
(June 1968) Ganymede
Eros o 1 Callis^to^
(May 1968^,^^/ Europa
<u
to
u
0)
&
o
a
e
•H
to
OJ
CO
CO
o
iruranus
I [Neptune]
P'Titan;
J" J"
Figure 16. Attenuation of signal strength
during its return to the radar equipment.
Along the abscissa is plotted the time
of the signal's motion at both ends, and
along the ordinate is plotted, on the
right, the attenuations in powers of ten,
and on the left, in decibels. Each
space object is represented by a line,
encompassing the entire distance of the
object from the Earth — from the nearest
to the remotest.
published in print, they have
not been confirmed. Up to the
present time, the growth in
sensitivity of radar devices on
Earth has been at a rate of 5.5
dB (or 3.5 times) a year. It
may be that a threshold corre-
sponding to a loss of 390 db .
will be reached in 1972. Then
it will be possible to have
accurate radar scanning of
Jupiter and even its satellites.
Radar scanning of Saturn will
also be possible.
Spectroscopy of the planets .
Great possibilities for spectral
analysis of the planetary at-
mospheres are revealed by the
method of Fourier-spectroscopy.
The principles involved in this
method can be understood if we
recall the operating principles
Light from the point source S
1'
of the Michelson interferometer (Figure 17)
with the aid of the collimator lens L travels in a parallel beam to the
separating plate P, mounted at an angle of 45°. Here the beam is split,
being reflected from the translucent reflecting plane of the plate P, and
partially travelling through it. The first beam encounters the mirror M,
and the second — the mirror M„. Reflected from them, the light again
encounters the plate P, being partially reflected from it, and partially
passing through it, after which it is directed toward the lens L , which
gathers both beams at the focal point F, where there may be an eye, a photo-
plate or a photomultiplier . The beams travelling toward L„ interfere with
one another. The focal point F will be "light" or "dark" depending on
/60
59
^
3 AC,
Figure 17. Schematic of the
Mlchelson Interferometer.
whether the path difference of the beams forms
an even or odd number of half-waves of mono-
chromatic light of a given wavelength. If the
light Is not monochromatic, but "white", then
It will always contain a wavelength A ' , which
will give "light", and along with it X", which
will result in a "dark" point A' and X" will
differ to a lesser extent, the greater the
path difference of the beams, reflected from
M^ and M , because with a large path difference
(due to the large number of half-waves con-
tained in it) even for A' and A" quite close,
there may be a difference up to a half-wave
Now If the mirror M moves at a constant rate v from the middle position /61
(for which the path difference is equal to zero) to the path difference t,
then at the point F each wavelength will be modulated at a different fre-
quency. The wavelength A = x will be modulated only one time, and the
smallest A of the wavelengths transmitted — x/A times. In the general
m m
case, since the path x will be travelled for a time x/v, the frequency of
modulation
/ = t/A, : T/o = o/A,
will also be different for the different wavelengths. At the same time the
Intensity of the radiation modulated at a frequency f will be a direct
function of the intensity of the radiation at the wavelength A = v/f . When
the photocurrent is recorded from the photocell at the focal point F, then
it expresses the effect of adding the emissions at all wavelengths of the
examined Interval. This would be like noise containing a vibration in the
Let us say the path difference Is 500A ' and then A" is determined
A' A"
from the equation 1000-r = 1001— r, which gives A' - A" = O.OOIA", whereas
with a path difference of 50A', we find A' - A" = O.OIA".
60
wide frequency band. But at each moment their frequencies and their wave-
lengths X participate in this noise and, moreover, at a different intensity,
corresponding to the intensity in the spectrum.
Since the examined noise is composed of many harmonic vibrations, all
of its "vibration" for the period of the photocurrent recording (when the
mirror of the interferometer moves) can be expanded into elementary harmonic
functions, for example, according to cosines of all the frequencies of
modulation — that is, it is subject to Fourier transformation. Such a
possibility (theorists and experimenters) was predicted for optics even at
the beginning of this century. But the practical mastering of this method
became an actuality only in recent years after overcoming certain specific
experimental difficulties, especially noticeable in astronomy, when the
image of an object flickers strongly due to the instability of the atmosphere.
Furthermore, the assistance of modern electronic computers has become possible,
since without them the tremendous computational effort associated with
Fourier transforms is simply inconceivable. It was further found that
spectroscopy with the aid of an interferometer has the advantage of a
greater luminosity versus the ordinary recording of the intensity of the
spectrum. This is especially important in astronomy where the light sources
to be investigated are very weak in comparison with those in the laboratory.
With the appropriate precautions, the use of this method will significantly /62
increase the accuracy of the measurements. Finally, the resolving power in
Fourier spectroscopy may be significantly higher than in the classical
(8)
method . This is especially apparent in the infrared band, but not because
the ordinary methods give a low resolving power in the spectrum (this is not
so), but because, due to the low sensitivity of the infrared light receivers,
the experimeter must register a wide band of the spectrum directly. Otherwise
As we can see from the footnote on page 60, the resolution is ob-
tained equal to 1/2t in the scale of wave numbers a (in cm ) . With the
2
aid of the relationship Aa/a = -AX/ A we find the expression AX = -X Act, which
shows that the resolution is improved with decrease in wavelength.
61
his light receiver will simply not respond to the incoming luminous flux.
The use of infrared receivers, which are distinguished by the fact that their
noise, as a rule, does not depend on the strength of the signal, makes
Fourier spectroscopy quite advantageous, especially when the problem is to
obtain a high resolution in the spectrum.
All these properties of Fourier spectroscopy are especially important
in the investigation of the planets, since the majority of molecules com-
prising planetary atmospheres are best seen in the infrared band with its
rotational-vibrational spectra.
All the small luminous fluxes reaching us from the celestial bodies
including the planets, when our purpose is to carry out precise measurements
of the intensity in the spectrum with a high resolving system, require a long
recording time even with large telescopes and with the use of Fourier
spectroscopy. Modern technical procedures — recording on punched tape or
on magnetic tape and the subsequent transmission of this recording by
telephone to a large computer center — make it possible with the least loss
of effort and time to obtain a final result which even 15-20 years ago
would have been impossible. Figure 18 shows a part of the spectrum of Venus,
obtained in this manner by Konn and Mayar on July 3, 1966 on the 193-cm
reflector of the observatory of Upper Province (France) in two approaches
when Venus was near the meridian (Venera-1) and far from it (Venera-2) . The
telluric lines, formed in the Earth's atmosphere, designated by the letter T,
are much stronger in the second case. For comparison the spectrum of the Sun /64
was recorded. All the lines, in the spectrum of Venus and absent in the
spectrum of the Sun, belong to the weak band of carbon dioxide around X =
= 2.2 y. The total observation time of the spectral region, which is twice
as wide as shown here, was 27 hours. The time for transmitting the data to
the computer center is somewhat less than this, but the computations themselves
occupy 1-2 hours.
62
iSOO
Z2ZV-
wo
ifSZO
&ZfV
ifSSBcM--
Figure 18. Part of the spectrum of Venus compared with the spectrum
of the Sun in the range of 2.22-2.23 y. The telluric lines (formed
in the atmosphere of the Earth) are denoted by the letter T. The
lines forming in the solar atmosphere are denoted by the letter S.
The others are formed in the atmosphere of Venus. The only differ-
ence in the spectra from Venera-1 and Venera-2 is that the second
was obtained at a lower position of Venus over the horizon. The
majority of the lines in the spectrum of Venus belong to the weak
band of carbon dioxide (CO.) .
Radioscopy of a Planet's Atmosphere . The previously mentioned radioscopy
of the atmosphere of a planet by the light of a star is a phenomenon that is
both unusual and difficult to observe.
This phenomenon does not depend on the will of man. Radioscopy of the
atmosphere of a planet by radio waves from a spacecraft orbiting the planet
is a much simpler affair (after the equipment reaches its target) , and it may
be organized just like any other physical experiment.
Let us acquaint ourselves with the principles of this experiment.
Figure 19a, shows the passage of light beams through the atmosphere of a
planet in simplified form, as though the atmosphere consisted of individual
layers, at the boundary of which the light is refracted, so that the angle of
refraction is less than the angle of Incidence if the light travels from a
less dense layer to a more dense one. The opposite picture is observed when
the light travels from the atmosphere. If we were to plot the refraction on
63
from star
an infinitely large number of
infinitely thin layers, the
trajectory of the beam in the
atmosphere would appear to be
curvilinear in precise agreement
with actuality. The described
phenomenon is called atmospheric
refraction in astronomy. As is
obvious from the drawing, the
initially parallel pencil of rays
becomes divergent due to the
refraction.
to Earth
c)
Figure 19. Schematic of the radioscopy
of the atmosphere of a planet by the
light of a star (b) and by radio sig-
nals from a spacecraft (c) . Refraction
in the atmosphere of the planet (a) .
Now let us look at Figure
19b, where the obscuration of a
planet by a star is shown. The
planet is first affected by the
star over its entire atmosphere.
The parallel pencil of light from
the star becomes divergent. When
one of these rays, or more
properly, a narrow pencil of rays,
reaches the observer located at a
distance L from the planet, the
observer sees the star, but its brightness will be weakened, because as a
result of refraction, the energy contained in the divergent pencil of rays
is less than before refraction in the parallel beam of the same cross section.
Such a refraction weakening is significantly greater than the weakening of
light by absorption in the atmosphere. At first, when the star is behind the
disk of the planet, light approaches the observer which has penetrated the
more rarefied layers of the atmosphere, and then the denser ones. The star
"darkens and goes out". This takes place more effectively, the greater the
distance L of the observer from the planet. A precise formula shows that the
weakening factor will be (1 + 2a)Lg) where 2a) is the angle at which the light
/65
/66
64
ray is deflected as a result of refraction when it passes through the
atmosphere of the planet (horizontally, in the lowest part of its trajectory) ,
and 3 is the characteristic of change in the density of the atmosphere with
altitude. An Increase of altitude of H = 1/3 km results in a density drop
of a factor of e. In the formula for the damping, only u is a variable
quantity, and the observer sees the progressive damping of the brightness of
the star in proportion to the growth in the horizontal refraction w. Knowing
very precisely the position of the planet and the star in space, and also
the radius of the planet, we can compute at any given moment of observation
how near the star approaches the planet's disk, and in the same manner we
may always know the angle u). Consequently, from the observations we can
determine the quantity 3 or its inverse scale of height H. But this quantity
is associated with the characteristics of the atmosphere — its molecular
weight VI » temperature T, and acceleration of the force of gravity g, by the
simple formula
where 9t is a universal gas constant. The quantity g is easy to compute.
Consequently, after determining H from the observations, we can find the
molecular weight y, if we know the temperature T. Conversely, we can
determine T if we know y.
Unfortunately, the temperature T is not constant in the atmospheres of
the planets, although in the upper stratospheric layers, it is rather in-
variable. The radii of the planets are known to us with an insufficiently
high degree of accuracy, so that slight uncertainty remains in determining
the level of the planet's atmosphere, to which these or other values of y or
T pertain. The divergence may reach several scores of kilometers for Venus
and hundreds of kilometers for Jupiter, but for an approximate determination
of the physical parameters of the planets' atmospheres the method of radio-
scopy is quite good.
65
One of its variations, radioscopy of the atmosphere by radio waves
originating from a spacecraft is shown schematically on Figure 19c. It
differs from the preceding case in that here we are studying the passage
through the atmosphere of a beam of radio waves which diverge from the point /67
source, the position of which M ,M„,... at different moments of time is known
precisely . Figure 19c shows only those rays originating from the craft in
positions M^ and M„, which have reached the observer on Earth. It seems that
(9)
here also refraction damping is also taking place, but in addition, there
is still another effect, that is, change in the frequency or wavelength of
the radio emission when it passes through the atmosphere. In general when
the spacecraft travels the trajectory M ,M , . . . , the frequency of the radio
signal varies as a result of the Doppler effect, because the rate of motion
of the spacecraft and the angle between the line of sight and the direction
of the motion both change. If the rate of motion in the projection on the
line of sight is v, and the propagation rate of the radio waves is c, then
the relative change Av in frequency v — that is, Av/v — is equal to the
ratio v/c. But the rate propagation of the radio waves differs in a vacuum
and in a refracting medium, whatever the atmosphere of the planet may be.
Therefore, the signal may arrive on Earth with a phase which differs from
that in the absence of an atmosphere. Taking into account all the sources
of change in frequency and time of propagation on the path from the space-
craft to the receiving antenna (including the role of the Earth's atmosphere) ,
we can compute for each moment the changes in the phase of the arriving
signal, and after comparing them with the observed changes and after de-
termining the divergence, we can attribute it to the effect of the planet's
atmosphere. The divergence directly influences the change in the coefficient
of refraction n, and this quantity — more precisely, its difference from
unity, (n-1) — depends directly on the density of the atmosphere and its
chemical composition. Thus, of course, after complicated treatment, we can
obtain the density distribution with respect to altitude, and, hence, it is
(9)
The picture of the refraction of radio waves in the atmosphere,
shown on Figure 19c, corresponds to short waves of the centimeter and
decimeter bands.
66
easy to convert also to temperature. This method gives more precise results
than measurement of the signal attenuation.
In the experiment of Mariner-4 in its orhit near Mars, phase shifts were
observed in the oscillations arriving both when the craft approached the
planet's disk and when it emerged from it. Quantitatively they agreed well
and during the time of the approach and emergence reached about 30 complete
cycles of oscillations, which converting to the quantity (n-1) comprised a
— fi
factor of 3.6 X 10 . Hence, the value of the atmospheric density at the /68
-5 3
surface of the planet is about 1.5 x 10 g/cm (under certain assumptions
concerning the chemical composition of the atmosphere) and the pressure varied
between 4 and 6 millibars which is found to be in satisfactory agreement with
the spectroscopic results. The scale height was found to be between 8 and 10
km. Under various assumptions on the chemical composition, the temperature
is found to vary from 170 to 180° K. The transmitter giving all this
information operated at a frequency of 2297 MHz and had a power of only 10
watts I Its distance from the Earth during this time was 216 million kilo-
meters.
In the experiment with Marlner-5 which completed flight around Venus
with a transmitter frequency of 2297 MHz, the phase shift reached 140 cycles,
and the changes in (n-1) comprised from 15 x 10 to 1464 x 10 for dis-
tances from 6123 to 6088 km from the investigated atmospheric layer to the
center of the planet. But what altitude above the level of the surface do
these distances represent? For an answer to this question, we must know
the radius of the planet's surface. Visual observations cannot give this
quantity for Venus, — it is determined only from radio observations and
more accurately, — from radar probes. Radar probes resulted in a radius
of Venus between 6050 and 6056 km. Consequently, the flight of Mariner-5
gave physical characteristics of Venus' atmosphere from an altitude of 70 km
to 32 km. The altitude scale was defined as 8.9 km for the lower boundary,
and the values of the temperature were found to be about 400° K and a pressure
of about 6 atm. The pressure and temperature pattern, obtained from this
experiment, agrees excellently with the pattern of these quantities in the
67
experiments with the unmanned spacecraft Venera-4 and Venera-5. If it is
applied up to the very lowest level of the atmosphere, with a radius of
6053 km, then figures are found that are similar to those given above, that
is, a temperature of about 770° K and a pressure of about 100 atm.
In the entire experiment of Mariner-5, precise knowledge of the entire
geometry of the phenomenon is of primary importance, that is, the mutual
distribution of the points M^ , M , ..., of Venus and the Earth, and the value
of the planet's radius. Change in frequencies of the signals is accomplished
with great accuracy up to 0.08 Hz, which made it possible to follow with high
accuracy the trajectory of the unmanned spacecraft Mariner-5 for the entire
trip. In this case celestial-mechanical computations were carried out in
parallel, especially when the spacecraft was nearing Venus, since Venus /69
produced a very substantial change in the orbit of the craft relative to the
Sun (its velocity relative to the planet grew from 3.05 to 8.56 km/sec, and
the frequency of the received signals changed by 95,000 Hz because of this).
American scientists consider that, as a result of all the measurements and
computations, the position of Mariner-5 relative to the center of Venus was
known for the entire approach time within an error no greater than 0.2 km.
But in order to obtain such high accuracy, the motion of Venus must be known
as precisely as possible. The necessary data, reinforced by the results of
radar observations, were given by celestial mechanics.
We should note the fact that in the radioscopy of the Martian atmosphere
it is not the signal itself from Mariner that was observed, but a retransla-
tion by it of the signal sent from Earth. This signal was regulated by the
oscillations of rubidium atomic source which ensured its superhigh stability.
In investigating the atmosphere of Venus, such a technique was not used,
since the double passage of the signal through its superdense atmosphere
threatened too large an attenuation of the signal's strength.
Greenhouse effect . The greenhouse effect of a planet's atmosphere, just
as in our hothouses covered with glass, is based on the fact that solar
68
radiation, heating the planet's surface, passes through the atmosphere
comparatively freely, and the radiation from the heated surface of the
planet cannot go beyond the limits of the atmosphere, since it is absorbed
by the gases which heat them. In our hothouses glass plays this role. It
does not prevent the short-wave solar radiation from penetrating inside the
hothouses, but it does restrain the radiation of heat from the hothouse to
the outside, since this radiation is of the long-wave type, and the glass
for the long-wave. Infrared radiation is opaque. Not every atmosphere
possesses such a restraining effect: for example, hydrogen, helium and
nitrogen atmospheres do not have this property.
Quite another situation is involved with an atmosphere containing carbon
dioxide, water vapors, and ozone. If we investigate the spectrum of any
source, the light of which has travelled a sufficiently long path in carbon
dioxide or in water vapors, then in the infrared band of the spectrum we will
detect a number of very dark bands, indicating the absorption of radiation. /70
These bands give information on the radiation at a wavelength of 1-4 y (mainly
water vapors) , but at 4 and 15 v carbon dioxide, and at 6 and 50 y — water
vapors almost completely retard the radiation. A planet radiates basically
between 10 and 15 y, if its temperature equals 250° K, and between 4 and 15 y
at a temperature of 600° K. Fortunately, for astronomy, these bands do not
merge with one another, if we examine the radiation of celestial bodies after
passing through the Earth's atmosphere. Spectral "windows of transparency"
are found in them, that is, wavelength intervals of the infrared band in which
the absorption is not high, and celestial bodies can be observed without
losses due to absorption.
However, the Earth's atmosphere is not rich with carbon dioxide and
water vapors. Venus is a different situation, where more than 90% of the
atmosphere consists of carbon dioxide, and the absolute content of water
vapor is significantly higher than in the atmosphere of the Earth. Thus,
all the absorption bands of these gases are merged together, and the natural
heat radiation of the planet, if it has a temperature of 200-700° K, cannot
exceed the limits of even the lowest layers of the atmosphere, — it will
69
,3
cloud
'^^ layer
|T=700°K
■yd \
k^Ett
P^-w^o -ha:?^ Jfifeav ;.,
Figure 20. Greenhouse effect In the atmosphere
of a planet rich with carbon dioxide and water
vapors. The solar energy flux arriving at a
given site on the planet is shown by the
light (wedge-shaped) band. In the cloud lay-
er a significant part of this energy is
scattered, which is shown by the decrease in
band width. This scattering continues farther
below the clouds due to the encounter of the
photons with the gas molecules and the solid
and liquid particles, suspended in the at-
mosphere. A very small part of the radiation,
which is absorbed here and heats the planet,
reaches the surface of the planet. Simul-
taneously with scattering in the atmosphere
an insignificant absorption takes place (the
gray band inside the light one) thus heating
the atmosphere. The heated surface of the
planet emits long-wave radiation which is
absorbed by the atmosphere as a whole (atten-
uation of the light points with altitude) ,
with the exception of the radio waves of 3-50
cm, which leave the atmosphere without hin-
drance (wavy lines) . Convective currents
(the weak spiral lines) also leave the hot
surface of the planet into the atmosphere.
A luminous energy flux
from the Sun reaches the highly positioned cloud layer of the planet. This
is shown by the light shape in the center of the drawing, the width of which
70
be absorbed. Of course, it
will be partially reradiated,
but a considerable part of
it will go into heating the
gases, and the reradiation
will go to all sides in-
cluding back to the surface.
As a result, the thermal
radiation flux going out-
side is decreased almost to
zero. But the heat flux
leaving the Sun does not
cease, the surface of the
planet is heated, and only
when equilibrium is estab-
lished between the freely
arriving heat and the natural
heat, which leaves the planet
"with difficulty", is there
a temperature equilibrium
established at a certain
comparatively high level.
Figure 20 shows this
with a schematic represen-
tation of the conditions of
heat transport in the at-
mosphere of a planet, as
applied specifically to
Venus .
J
I
' corresponds to the strength of the flux. After encountering the cloud cover,
the solar light is scattered by the clouds. The scattering is very great.
It continues also below the clouds where the flux undergoes strong attenuation, /71
; and is propagated farther down, the greater it is scattered, creating a
I luminous field similar to that which exists on Earth on a cloudy day (this is
' shown on the drawing by an overall semi-light background) . Let us recall that
I the scattering involves the scattering of atoms, molecules, and particles
; reradiating the incoming photons, without changing their frequencies. The /72
situation is that the almost unchanged solar light, scattered by the clouds or
penetrated from under the cloud layer, is emitted back into space by Venus, —
about 75% of all the light incident on it. If we speak about all the solar
energy, including the infrared and the ultraviolet bands of the spectrum, then
for the planet there remains only 28%; and 72% is reflected and lost irrevocably.
There 28% are absorbed, that is, they either go into restructuring the internal
structure of the atoms or molecules, and then into heating the gas, or —
unlike scattering — the absorbed photons are reradiated at another wavelength,
but such reradiated photons are recaptured and also go into heating the gas.
This process of progressive absorption is shown on Figure 20 by the tapering
gray wedge shown inside the outer light wedge of the scattered energy flux
reaching the surface of the planet in a negligible amount.
At the same time, the hot surface of the planet radiates a rather large
amount of energy (the light points on Figure 20) . The upward energy flux is
very rapidly attenuated because of absorption in the carbon dioxide and water
vapors. Nothing reaches the cloud layer; even convection (on Figure 20 the
spiral columns) is in no position to help transport the heat. Therefore,
measurements of the natural heat flux from Venus in the infrared band (after
subtracting the solar energy flux reflected from the clouds) leads to a
temperature of 240° K, which we attribute to the cloud layer. Only radiation
through radio waves in the band from 3 cm to 50 cm reaches us unimpeded from
the surface. Neither the dense atmosphere nor the clouds affect it, and,
therefore, with their help the real temperature on the surface of Venus is
about 700° K.
71
TABLES OF PHYSICAL CHARACTERISTICS OF THE
MAJOR PLANETS AND THE MOON
Mean distance from the Sun
Eccentricity of orbit
Inclination of orbital plane to ecliptic
Rotation period around the Sun
Synodic rotation period
Mean rate of motion in orbit
Diameter from radar measurements
Diameter from optical measurements
Angular diameter, seen from the Earth:
(a) when Mercury moves along the
Sun's disk ) in November
MERCURY
a
0.387 A.u/*'
e
0.206
liptic
1
7°0'15"
P
88.0 days
S
115.9 days
V
47.9 km/ sec
°0
4860 + 4 km
D
4850 + 40 km
in May
(b) in mean (by distance) elongation
Area of disk in average elongation
Area of disk, visible from Sun at mean
distance from it
Mass in solar masses
Mass in Earth masses
Absolute mass
Volume in Earth volumes
Mean density
Rotation period around axis (stellar days)
Inclination of equator to orbital plane
Moment of inertia
Acceleration of force of gravity at
equator
d"
9.8"
d
12,1"
d
7.3"
to
-9
I'lO sterad
Q
1.3 '10 sterad
m
1:6021000
gji
0.956
aji
3.303-10^^ g
p
0.0553
5.59 g/cm^
p'
58.65 days
1'
7°
unknown
8„
372 cm/sec
111
(*)
A.U. = astronomical unit, equal to the mean
Sun; 1 A.U. = 149,600,000 kilometers.
distance of the Earth from the
72
Acceleration of force of gravity in units of
Earth acceleration
Critical (parabolic) velocity at which a
body leaves the planet
Stellar magnitude during the time of the
mean(by distance) superior conjunction in
the V system.
Stellar magnitude in elongation (as a func-
tion of distance from the Sun) in the
V system.
Index of yellowness (excess over the color
index of the Sun)
in the system B-V
in the system B-I
Visual spherical albedo
Thermal spherical albedo
Equilibrium mean temperature at a mean
distance from the Sun
Equilibrium mean temperature for the
subsolar point (computed)
From measurements in infrared rays
Dark side has a temperature no higher than
In the microwave region, the following
mean brightness temperatures over the
disk (at a mean distance from the Sun)
are measured at :
X = 0.34 cm
0.86 cm
1.53 cm
3.5 cm
10 . 6 cm
11.3 cm
Dependence of T, on phase
Atmosphere
The amount of CO. determined at the limit
V
0.38
4.3 km/sec
-i^'.yi
from
-0'".3 to +0'^.6
\
comp
+0"'.30
+0°^.93
0.056
0.09
505° K
618°
K
613°
K
250°
K
200 - 220° K
404 ± 40°
465 + 115°
390 + 100°
290 + 40°
290 + 40°
unconfirmed
unconfirmed
from 1.5 to 3.5 m-atm
i74
N and Ar and, in the upper layers — CO and as a result of dissociation
2 ^
of carbon dioxide are also theoretically possible. Mercury has no satellites.
73
VENUS
Mean distance from Sun
Eccentricity of orbit
Inclination of orbital plane to plane of
ecliptic
Rotation period around Sun
Synodic rotation period
Mean rate of motion in orbit
Diameter over surface
Diameter over level of cloud layer
Diameter over level of obscuration of
Regulus
Angular diameter seen from the Earth
in inferior conjunction
in superior conjunction
Area of disk seen from the Earth:
in inferior conjunction
in superior conjunction
Area of disk seen from the Sun at a mean
distance from it
Mass in solar masses
Mass in Earth masses
Absolute mass
Volume in earth masses
Mean density
Rotation period around axis (stellar
days); retrograde rotation
Rotation period of visible surface (cloud
layer) ;retrograde rotation
Inclination of equator to orbital plane
Moment of inertia
Acceleration of force of gravity at
equator
a
0.723 A.U.
e
0.007
1
3°23'40"
P
224.7 days
S
583.9 days
V
35.0 km/ sec
°o
12105 ±4 km
D
12200 ±20 km
D' 12338 ±10 km
d 60.8" (max. 65.2")
d 9.8" (min. 9.5")
—8
03 6.8-10 sterads
-9
(0 1.8 "10 sterads
—8
1.0* 10 sterads
1.408250
0.815
27
gjl 4.867-10
Vq 0.861 g
P 5.22 g/c
P' 243.0 -0.5 days
P" 4 days
t' 178° (*)
unknown
2
g 886 cm/sec
III
(*)
This angle is equal to 2 , but the directions of the axial and orbital
motions are opposite to one another.
74
Acceleration of force of gravity In units
of earth acceleration
Critical (parabolic) velocity at which a
body leaves the planet
Stellar magnitude during time of mean (by
distance) superior conjunction in the
system V
Stellar magnitude during time of inferior
conjunction when the planet is located
precisely between the Sun and the Earth.
The same near inferior conjunction
Maximum brightness (before or after inferior
conjunction of 35 days)
Index of yellowness (excess over color index
of Sun) in the system
Visual spherical albedo
Thermal spherical albedo
Equilibrium mean temperature (computed)
Actually abservable temperature of cloud
layer over the disk (from Infrared
measurements) without confirmed differ-
ence in night and day side
Measured in the microwave region at:
X <_ 0.3 cm
X >_ 2 cm
X >^ 21 cm
By direct measurement from unmanned
spacecraft Venera-4, Venera-6, and
Marlner-5 of the surface temperature
No confirmed indications that the night side
of Venus is colder than the day side
Chemical composition of the atmosphere from
unmanned spacecraft Venera-4 and Venera-6:
carbon dioxide C0„
Be'
0.90
V
e
10.3 km/sec
-S^'.Sl
-0"^.l
-s'^.s
-4". 45
B-V
-0^19
A
V
0.76
h
0.77 + 0.07
229° K
comp
nitrogen N~
water H„0
other gases: CO, O™, HCl, HF
225 - 235 - 240° K
^ 300° K
^ 700° K
<. 600° K
'^> 750° + 100° K
97 + 4%
no more than 2%
0.05%
admixtures
75
Atmospheric pressure on surface
Altitude of uniform atmosphere:
at its base
at level of obscuration of Regulus
Optical thickness of cloud layer in visible
band
Cloud layer has large heterogeneities
(partial gaps)
Maximal electron concentration in the
ionosphere at an altitude of about 90 km
over the cloud layer, by day (by night
it is 50 times smaller)
100 + 40 atm
H 13 km
H 6.8 km
Tq 70 + 40
m.
n 5.5-10^ cm ^
e
No magnetic field was detected on Venus (as the dipole is 3000 times
weaker than on Earth)
EARTH
Mean distance from Sun
Eccentricity of orbit
Rotation period around Sun
Mean rate of motion in orbit
Equatorial diameter
Polar diameter
Mean diameter
Flattening e = (D^ - D^) rD^
Area of disk visible from the Sun at an
average distance from it
Mass in solar masses
Absolute mass
Volume
Mean density
Rotation period around axis (stellar days)
Inclination of equator to orbital plane
(ecliptic)
a
1.000 A.U.
e
0.017
P
365.256 days
V
29.8 km/ sec
°E
12,756.3 km
°P
12,713.6 km
D
12742.1 km
e
1 : 298.2
fi
0.57-10"^ sterad
SW
1 : 332944
m.
5.976-lO^^g
!o
p
1.083-10^^ cm-^
5.517 g/cm-^
p
23 hr. 56 mln. 4.099 sec
23° 27'
76
Moment of Inertia (in units ofTOR ) I
Ratio of centrifugal force to force of gravity
at equator $
Acceleration of force of gravity at equator
Critical (parabolic) velocity at which a body
leaves the planet
Stellar magnitude seen from the Sun in the
system V
Index of yellowness (excess over color index
of Sun) in the system
Visual spherical albedo
Mean temperature over surface of Earth
Maximal temperature for the subsolar point
The Earth radiates into space as an absolute
black body with a temperature of
Atmosphere of the Earth
nitrogen
oxygen
argon
water
carbon dioxide
neon
methane
other gases in the form of impurities
as a whole less than
Atmospheric pressure at sea level
Altitude of uniform atmosphere
Magnetic field: dipole moment
horizontal component
vertical component
((|) - geomagnetic latitude)
S.
V
B-V
rad
^2
°2
Ar
CO 2
Ne
CH,
H
Z
0.334
0.0035
978.044 cm/sec
11.2
km/ sec
-3"^.
87
-0^.
6
0.39
285°
K
349°
K
250° K
62,500 cm-atm
16,800 cm-atm
7440 cm-atm
from 3000 to 5000 cm-atm
220 cm-atm
14 cm-atm
1.2 cm-atm
2 cm-atm
1 atm = 1013.25 mbar =
1033.23 G/cm^
8 km
.25
8.06*10 elem . units.
0.315 cos ()> gauss
0.630 sin (j) gauss
77
MRS
Mean distance from Sun
Eccentricity of orbit
Inclination of orbital plane to ecliptic
plane
Rotation period around Sun
Synodic rotation period
Mean rate of motion in orbit
Equatorial diameter
Polar diameter
Flattening e = (D^ - Dp) :Dp
Djniamically determined flattening
Angular diameters, seen from Earth during
time of mean (by distance) opposition:
equatorial
polar
Area of disk in mean opposition
Area of disk visible from Sun at mean
distance from it
Mass in solar masses
Mass in Earth masses
Absolute mass
Volume in Earth volumes
Mean density
Rotation period around axis (stellar days)
Inclination of equator to orbital plane
2
Moment of inertia (in units of5DlR )
Ratio of centrifugal force to force of
gravity at equator
a
1.524 A.U.
e
0.093
i
1°51'0"
P
1.881 years
S
779.9 days
V
24.1 km/sec
°E
6,800km (*)
°P
6746 km (*)
e
1 : 125 (*)
1 : 190
17.9"
17.76"
—8
0.6-10 sterad
n
0.7 '10 sterad
Ti
1 : 3111000
m
0.1078
m
6.443-10^^ g
!o
p
0.150
3.97 g/cm^
p'
24 hr. 37 min. 22.668 sec
. T
1
23°59'
I
0.389
0.0043
^ ^The values of D and D shown here are derived from optical measurements
with a maximal error of + 20 km. From them the value of the mean diameter
(2 D„ + D„) ; 3 = 6769 km is found to be in good agreement with the mean value
E P
of 6758 km derived from radio eclipse by Mariner-4. But the value of flat-
tening computed from them is high, and contradicts the value determined
dynamically from the motion of the satellites.
78
Acceleration of force of gravity at equator
The same In units of Earth acceleration
Critical (parabolic) velocity at which a
body leaves the planet
Stellar magnitude during time of mean
opposition in the system
In superior conjunction with the Sun,
the planet is weaker by
Index of yellowness (excess over color
index of Sun)
in system B-V
in system U-I
Visual spherical albedo
Thermal spherical albedo
Equilibrium temperature at mean distance
from Sun
The same for the subsolar point (computed)
From measurements in the infrared band, the
mean brightness temperature over the disk
and the temperature of the subsolar point:
at mean distance from Sun
in perihelion
in aphelion
Mean brightness temperature over disk
from measurements in microwave region
at:
X = 0.34 cm
3.15 cm
6 cm
10 cm
21 cm
Chemical composition of atmosphere:
carbon dioxide
carbon monoxide
water: by precipitation from
atmosphere a layer Is formed with
a thickness of
8e
372 cm/ sec
8e
0.380
V
e
5.03 km/ sec
V
-2"'.01 ,
AV
3^.41
+0"^.
71
+1"^.
93
A
V
0.16
h
0.26
+ 0.05
comp
^0
216°
306°
K
K
^b
225°
K
286°
K
300°
K
273°
K
190° + 40° K
218° + 50° K
192° + 28° K
177° + 17° K
190° + 40° K
CO2 75+15 m-atm
CO
traces
H2O 35 y
79
In the upper atmosphere, as a result of
dissociation, atoms of H, 0, C
Other possible, but unobserved components N„ , Ar
Atmospheric pressure on surface p 20 > p > 6 mbar /yg
Altitude of uniform atmosphere H 13 km
Ionosphere only on day side with maximal c 3
concentration of electrons n 1.6 • 10 cm
e
Magnetic field None observed
Mars has two satellites, Phobos and Deimos, with a diameter of 15 - 10 km,
moving in the equatorial plane of the planet very near it (at distances of
9.37 and 23.52 thousand kilometers), with a period, respectively, of 0.319 and
1.262 days. In mean opposition they appear as objects of 11™ - 12™.
Mean distance from Sun
Eccentricity of orbit
Inclination of orbital plane to plane of
ecliptic
Rotation period around Sun
Synodic rotation period
Mean rate of motion in orbit
Equatorial diameter
Polar diameter
Flattening e = (D^ - D^) rD^
Dynamically determined flattening
Angular diameter, seen from Earth in
mean opposition:
equatorial d„ 46.5"
E
polar dp 43.7"
Area of disk seen from Earth in mean
opposition u 1.6'10~ sterad
The same from the Sun at mean distance
from it a 1.04'10~ sterad
80
JUPITER
a
5.203 A.U.
e
0.048
ane of
1
1°18'17"
P
11.862 years
S
398.9 days
V
13.1 km/sec
h
141,700 km
°P
133,100 km
e
1 : 16.5
1 : 15.34
Mass in solar masses
Mass in Earth masses
Absolute mass
Volume in Earth volumes
Mean density
Rotation period of visible surface (cloud
layer) :
in the latitudinal limits +12°
for the middle latitudes
Rotation period of decimeter radiation
carriers
The magnetic axis rotates with the same
period relative to the axis of visible
rotation; angle between them
Inclination of equator to orbital plane
2
Moment of inertia (in units ofOTR )
Ratio of centrifugal force to force
of gravity at equator
Acceleration of force of gravity at
equator
The same in units of Earth acceleration
Critical (parabolic) velocity at which a
body leaves the planet
Stellar magnitude during time of mean
opposition in the system V
In superior conjunction with the Sun, the
planet is weaker by
Index of yellowness (excess over color index
of Sun)
in system B-V
in system U-I
o
Visual spherical albedo (X = 5500 A)
Thermal spherical albedo
Equilibrium mean temperature over disk
(computed)
an
fat
^0
11
AV
h
comp
1 : 1047.39
317.82
30
1.899-10''" g
1347.0
1.30 g/cm^
9 hr. 50 min. 30.000 sec.
9 hr. 55 min. 40.632 sec.
Ill 9 hr 55 min 29.37 sec.
10°
i'
3° 04'
I
0.26
0.084
2.301 cm/sec
2.35
57.5 km/sec
-2"^. 55
0"^.85
0"^.20
0"^.27
0.67
0.45
110° K
81
Actually observed brightness temperature on
disk from measurements:
in infrared band
in 8 - 14 p band
in 10 - 14 jj band (color)
From measurements in the microwave region
at X = 0.2 cm the brightness is
2 cm
8.6 cm
21 cm
Over the rotational band, methane molecules
X =11, 070 A; the rotation temperature
Chemical composition of the atmosphere above
the cloud layer:
molecular hydrogen (spectrally)
helium (theoretically)
methane (spectrally)
ammonia (spectrally)
Altitude of uniform atmosphere
rot
«2
He
NH„
126° + 2° K (1964)
128° (1963)
125°
170° + 80° K
150°
149° + (1967)
400° ?
200° + 20° K
180° + 20° K
85 + 15 km-atm
26 km-atm
100 m-atm
5 m-atm
8 km
Jupiter has 12 satellites of which four (the Galilean satellites) are
large celestial bodies comparable in size to the inner planets and the Moon.
The others are small with diameters on the order of hundreds of kilometers
or less. The nearest, Amalthea, revolves around the planet in 2.5 days,
and the remotest, the ninth, in 758 days. Satellites 8, 9, 11, and 12 have
a direction of orbital motion, opposite that of Jupiter around its axis and
around the Sun.
SATUBN
Mean distance from Sun
Eccentricity of orbit
Inclination of orbital plane to plane
of ecliptic
a
e
9.539 A.U.
0.056
2°29'22"
181
82
E
Rotation period around Sun
Synodic rotation period
Mean rate of motion In orbit
Equatorial diameter
Polar diameter
Flattening e = (D„ - D ) : D
Dynamically determined flattening
Angular diameter, seen from Earth in mean
opposition:
equatorial
polar
Area of planet's disk seen from Earth in
mean opposition
The same from the Sun at mean distance
from it
Mass in solar masses
Mass in Earth masses
Absolute mass
Volume in Earth volumes
Mean density
Rotation period of surface (cloud layer) :
in latitude range + 25° - 30°
at other latitudes
Inclination of equator toward orbital plane
. 2
Moment of inertia in units ofSKR
Ratio of centrifugal force to force of
gravity at equator
Acceleration of force of gravity at equator
The same in units of earth acceleration
Critical (parabolic) velocity at which a
body leaves the planet
Stellar magnitude (with the exception
of the brightness of the ring) in mean
opposition in the system
In superior conjunction with the Sun the
planet is weaker by
p
29.458 years
s
378.1 days
V
9.6 km/sec
°E
120,670 ± 600 km
°P
109,110 ± 600 km
e
1 : 10.4
1 : 10.2
19.5"
17.6"
.-8
0.63*10 sterad
-8
n
0.51-10 " sti
an
1 : 3500.5
an
95.112
m
5.684-10^^ g
!o
p
770.5
0.71 g/cm^
P 10 hr. 14 mln.
P^ 10 hr. 40 mln.
1' 26°44'
I 0.21
$ 0.142
2
g 944 cm/ sec
g 0.965
V 37 km/sec
e
V +0™.67
AV +0^.46
83
The ring introduces additional brightness
which, in stellar magnitudes, is expres-
sed by the terms + 0.44 (j) - 2.60 sin B +
1.25 sin B, where (f> is the phase angle,
B is the angular elevation of the Earth
above the plane of the ring.
At maximal opening of the ring, that is,
when B = 28° , the stellar magnitude
of the planet is smaller by
Yellow index (excess over AV by the color
index of the Sun) in the system
Visual spherical albedo
Mean equilibrium temperature over disk
Brightness temperature from measurements
in the infrared band (different authors)
From measurements in the microwave region at
X = 0.86 cm
1 . 53 cm
3 . 45 cm
6 cm
10 cm
21 cm
Chemical composition of atmosphere above
cloud layer:
molecular hydrogen
methane
no helium observed
ammonia
Rings of Saturn
AV
B-V
comp
«2
CH,
He
0"^.95
f0'".41
0.69
80° K
from 85° to 125° K
96° + 20° K
146° + 23° K
106° + 21° K
217° + 30° K
196° + 44° K
303° + 50° K
'V' 40 km-atm
'^ 350 m-atm
undoubtedly exists
unconfirmed
Ring A, outer, brightness moderate at dis-
tance from center of planet Cassini scale
Ring B, middle, brightest, at distance
from center of planet - Dark space
Ring C, inner, dark (rigid, inner boundary
not sharp) , at distance from center
of planet of
from 138 to 120 thousand
km
from 116 to 90 thousand
km
from 89 to 71 thousand km
84
Thickness of rings
Hypothetical mass of rings In Satumi masses
2.8 + 1.5 km
10"^ - 10-5
Saturn has ten satellites of which only one. Titan, possesses planetary
-4
dimensions (D = 4950 km) and significant mass (2. 4 "10 of the planet's
mass). The nearest, Janus, (discovered in 1966) moves around the planet at
a distance of 157.5 thousand kilometers with a period of 18 hours and its
diameter is estimated to be 350 km. The remotest, Phoebe, revolves around
the planet for 550 days in the retrograde direction, at a mean distance of
13 million kilometers.
URANUS
Mean distance from Sun
Eccentricity of orbit
Inclination of orbital plane to plane of
ecliptic
Rotation period
Synodic rotation period
Mean rate of motion in orbit
Equatorial diameter
Polar diameter
Flattening e + (D^ - D^,) : D^
Angular diameter seen from Earth in mean
opposition:
equatorial
polar
Area of disk, seen from Earth in mean
opposition
The same from the Sun at mean distance
Mass in solar masses
Mass in earth masses
Absolute mass
a 19.182 A.U.
e 0.047
1
0°46'23"
p
84.015 years
S
369.7 days
V
6.8 km/sec
\
49,130 ± 100 km
°P
48,200 ± 1000 km
e
1 : 53
"^E
3.73"
S
3.66"
(0
2. 5 •10"''"° sterad
Q
2.3-10""'"° sterad
SDl
1 : 22934
n
14.517
ftn
8.676-10^^ g
85
_0
p
p'
i'
I
55.9
1.47 g/cm
10.8 hours
98° (*)
0.236
2
967 cm/sec
0.99
21.6 km/sec
+ 5"',52
- 0'
.m
.m
Volume in Earth volumes
Mean density
Rotation period around axis (stellar days)
Inclination of equator to orbital plane
2
Moment of inertia (in units ofOTR )
Acceleration of force of gravity at equator
The same in units of Earth acceleration
Critical (parabolic) velocity at which a body
body leaves the planet
Stellar megnitude during time of mean
opposition in the system V
Yellow index (excess over color index of
Sun)
In system B-V
in system U-I
Visual spherical albedo
Mean equilibrium temperature over disk
Temperature measured in the infrared
region in the range of 17.5 - 25 y
Brightness temperature measured in micro-
wave region at
X = 1.9 cm
3.75 cm
6 cm
11.3 cm
Rotational temperature in CH, absorption band
The same in the H absorption band
Chemical composition of atmosphere:
large amount of hydrogen
theoretically large amount of helium
7*)
The^direction of the axial rotation is retrograde; therefore, we assume
comp
07
- 1"', 62
0.93 (**)
60° K
55° + 3° K
^b
220° + 35° K
130° + 40° K
100° + 35° K
128° + 40° K
\ot
63° + 10° K
T
rot
124 ± 30° K
^-2
(**)
The largest of the planets in the solar system.
86
abundance of methane
Pressure at the boundary of the cloud layer
CH, from 3 to 150 km-atm, based
on various estimates of the
thickness of the CH^ ab-
sorption band in the
planet's spectrum.
p 3 atm
Uranus has five satellites. All of them are small (from 100 to 500 km
in diameter). They move practically in circular orbits, lying almost in the
planet's equatorial plane with periods from 1.4 to 13.5 days; the direction
of motion of the satellites coincides with the direction of Uranus ' rotation
— that is, it is retrograde.
NEPTUNE
Mean distance from Sun
Eccentricity of orbit
Inclination of orbital plane to plane of
ecliptic
Rotation period around Sun
Synodic rotation period
Mean rate of motion in orbit
Diameter from optical measurements
Diameter at level of half of loss of bright-
ness of the star during obscuration by
Neptune
Flattening
Dynamically determined compression
Angular diameter, seen from Earth in
mean opposition
Area of planet's disk seen from Earth in
mean opposition
The same from the Sun at mean distance
Mass in solar masses
Mass in Earth masses
a
30.057 A.U.
e
0.009
i
1°46'22"
P
164.788 years
S
367.5 days
V
5.4 km/sec.
D
47,000 +2000 km
D' 50,450 ±60 km
e 1 : 48
e 1 : 58
d 2.24"
0) 0.93-10""^'^ sterad
a 0.86-10~"'-° sterad
«Dl 1 : 19340
a» 17,216
87
Absolute mass
Volume in Earth masses
Mean density
Rotation period around axis (stellar days) P' 15.8 hours /85
Inclination of equator to orbital plane
2
Moment of inertia (in units of 9JtR )
Ratio of centrifugal force at equator to
force of gravity
Acceleration of force of gravity at equator
Same in units of Earth acceleration
Critical (parabolic) velocity at which a
body leaves the planet
Stellar magnitude during time of mean
opposition in system V
Yellow index (excess over color index of Sun)
in system B-V
in system U-I
Visual spherical albedo
Mean equilibrium temperature over disk
Mean temperature measured over the disk
at a wavelength of X = 3.12 cm
From obscuration of star at level D'
Significant part of atmosphere composed of:
hydrogen
methane
helium theoretically
Altitude of uniform atmosphere
Neptune has two satellites. The nearer, Triton, has planetary dimensions
and moves around the planet in a retrograde direction at a distance of 15.85
radii with a period of 5.88 days. The other, Nereid, is very small. It
revolves around the planet in a posigrade direction at a distance of 250 of
its radii, with a period of 359 days.
as
1.029-10^^ g
^0
p
50.65
1.88 g/cra^
p'
15.8 hours
i'
29°
I
0.241
$
0.022
Se
1194 cm/sec
Se
1.258
V
e
25 km/sec
+7'".84
-o'^.is
-2'".02
A
V
0.84
T
comp
51° K
^b
115° + 36° K
T
110° - 130° K
«2
CH^
'V 5 km-atm
He
H
50 - 60 km
88
■■■■■IIII^II^HH
Pluto
Mean distance from Sun
Eccentricity of orbit
Inclination of orbital plane to plane of
ecliptic
Rotation period around Sun
Synodic period of rotation
Mean rate of motion in orbit
Diameter
Angular diameter seen from Earth at mean
opposition
Area of planet's disk seen from Earth in mean
opposition (practically the same as from the
Sun)
Mass in solar masses
Mass in Earth masses
Absolute mass
Volume in Earth volumes
Mean density
Rotation period around axis (stellar days)
Stellar magnitude during time of mean
opposition in system V
Yellow index (excess over color index of Sun)
in system B-V
in system U-I
Visual spherical albedo
Mean equilibrium temperature over disk
(*)
.(*)
.(*)
(*)
D
V
0)
(A*)
(**)
(**)
(it*)
P'
(A* )
39.750 A.U.
0.253
17°8'.5
250.6 years
366.8 days
4.7 km/sec
6000 km
0".23
I-IO""""^ sterad
1 : 1,812,000
0.18
1.08-10^^ g
0.096
3
10.4 g/cm
6.39 days
+ 14.90
+0"^.17
+0"^.17
V
comp
0.14
43° K
786
(*)
It varies significantly for short periods of time as a result of strong
perturbations from other planets.
Very unreliable figure. From the nonoccuring obscuration of the star,
located very near the visible path of the planet, the upper limit was deter-
mined for the diameter D = 5500 km (but also with a large error) and then
p = 12.4 g/cm3. However, the mass is not kno^^m accurately (from perturbations
in the motion of Neptune) and may be two times smaller than the figure given.
89
Atmosphere
Satellites
unknown
unknown
THE MOON
a
384,400 ton
e
0.055
1
5°8'43.4" (*
P
27.3217 days
S
29.5306 days
V
1.023
D
3476.0 km
D'
0.2725
d
31'5.6"
0.00905 rad
(**)
6.45*10 ^ sterad
-9
Mean distance from Earth
Eccentricity of orbit
Inclination of orbit to plane of ecliptic
Stellar period of rotation
Synodic period of rotation
Mean rate of motion in orbit
Diameter
Diameter in units of Earth equatorial
diameter
Angular diameter at mean distance from Earth
or
Area of disk visible from Earth at mean
distance
Same visible from Sun at mean distance
from it
Mass in solar masses
Mass In Earth masses
Absolute mass
Volume in Earth volumes
Mean density
Rotation period around axis (stellar days)
Inclination of equatorial plane to plane
of ecliptic
Mean inclination of equatorial plane to
orbital plane
^ Afith respect to the Earth's equator, the inclinati«n of the Ixmar •rblt
varies between 23°27* ± 5°9', that is, from 18*18' to 28*36'.
'Because of perturbations from the solar side, it varies within limits of
of seven hours.
Because of eccentricity of orbit, it varies within limits of 13 hours.
90
Q
0.42-10 sterad
fUi
1 : 27069400
OT
1 : 81.3030
5W
7.35-10^^ g
!o
p
0.020
3.35 g/cm^
p'
27.3217 days
i"
1°33'
i'
6°41'
131
Moment of inertia (in units of R'^)
Acceleration of force of gravity on surface
Same in units of Earth acceleration
Critical (parabolic) velocity at which a
body leaves the Moon
Integral brightness at mean half-moon in
system V
Yellow index (excess over color index of Sun)
in system B-V
in system U-I
Visual spherical albedo
Temperature of subsolar point '
From measurements in the infrared band
Rapid cooling during lunar eclipse up to
Two days after onset of night up to
Temperature at midnight at the equator
Mean temperature over the entire surface
Actually measured brightness temperature
in the microwave region range from 185°
to 270° with no explicit dependence on
wavelength
Amplitude of radio temperature oscilla-
tions during lunar days (synodic
rotation period) varies from
from (at X = 0.13 cm)
to (at A = 10 - 20 cm)
and is practically equal to zero at
A > 30 cm
Atmosphere on Moon in Earth units no
greater than
Magnetic field on lunar surface
Magnetic moment
or in Earth units
I
0.39
g
162.0 cm/sec
g
0.166
V
e
2.37 km/sec
-12"*. 74
+0"'.29
+l"'.29
A
V
0.067
comp
387° K
371° K
175° K
(1939)
T'
122° K
(1963)
\
115° K
274° K
comp
^•t given/
120 - 115° K
5 - 7° K
-12
10
< 4y
20
< 10 erg/gauss
<io-6
Translated for National Aeronautics and Space Administration under contract
No. NASw 2035, by SCITRAN, P.O. Box 5456, Santa Barbara, California, 93108
91
NASA-Langley, 1972 30 F-698
M
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